Subnets External and Internal View of a Network Data comes into the router in the form of a data link frame (for now). Router strips off data link header. Router examines network layer address (IP Destination address) Router consults its routing table to determine which port to send out the data link frame. On the Internet (and other Wide Area Networks), "exterior" routers see each other as single networks and have no knowledge of their internal structure (including hosts and subnets). This keeps routing tables relatively small, like calling the Brady Bunch. Internally, networks may view themselves differently than the rest of the world. For greater flexibility, the network administrator may divide their single network (single IP network address) into several subnetworks or subnets. Internet Private network Router 130.5.32.0 130.5.64.0 130.5.96.0 130.5.224.0 . . . Subnetting 1985, RFC 950 defined a standard procedure to support the subnetting or division of a single Class A, B, or C network number into smaller networks. Problems subnetting was meant to solve: Internet routing tables becoming too large. Local network administrators had to request another network number from InterNIC before a new network could be installed at their site. Page 1 of 29 E:\161\Subnets.doc
Transcript
Subnets
External and Internal View of a Network Data comes into the router in the form of a data link frame (for now). Router strips off data link header. Router examines network layer address (IP Destination address) Router consults its routing table to determine which port to send out the data link frame.
On the Internet (and other Wide Area Networks), "exterior" routers see each other as single networks and have no knowledge of their internal structure (including hosts and subnets). This keeps routing tables relatively small, like calling the Brady Bunch. Internally, networks may view themselves differently than the rest of the world. For greater flexibility, the network administrator may divide their single network (single IP network address) into several subnetworks or subnets.
Internet
Private networkRouter130.5.32.0
130.5.64.0
130.5.96.0
130.5.224.0
.
.
.
Subnetting 1985, RFC 950 defined a standard procedure to support the subnetting or division of a single Class A, B, or C network number into smaller networks. Problems subnetting was meant to solve: Internet routing tables becoming too large. Local network administrators had to request another network number from InterNIC before a
new network could be installed at their site.
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Hierarchy Classful addressing provided a two-level hierarchy:
Two-Level Classful Hierarchy
Network-Prefix Host-Number
Three-Level Classful Hierarchy
Network-Prefix Subnet-Number Host-Number
Network Administrator "borrows" bits from the Host-Number to form: Subnet-Number HostNumber
Subnetting addressed the problem of expanding routing tables by ensuring that the subnet structure is not visible outside of the organization's private network. Internet routers see all subnets of a single network as a single network and keeps only a
single entry within its routing table. (see previous diagram) Only the router that connects the subnetted network to the external network (Internet) and the
"interior" routers see networks divided into subnets by their network prefix. Routers within a subnetted environment use the Extended-Network-Prefix to route traffic Extended-Network-Prefix = Network Prefix + Subnet-Number
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The Subnet Mask Extended-Network-Prefix (traditionally) identified by the subnet mask. Subnet mask is used to tell devices which part of an address is the network-prefix including the Subnet-Number, and which part is the host. Subnet Mask separates Network/Subnet address from Host address Subnet Mask (Cisco) = 32 bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as a mask. Subnet Mask 32 bits long Divided into four octets Network and Subnet portion -> all 1's Host portion -> all 0's
Extended-Network-Prefix
Network-Prefix Subnet-Number Host-Number
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Default Subnet Masks for Classful Networks (No subnets used)
Class A 11111111.00000000.00000000.00000000 255.0.0.0 Network.Host.Host.Host (Just like previous lesson)
Class B 11111111.11111111.00000000.00000000 255.255.0.0 Network. Network.Host.Host
Class C 11111111.11111111.11111111.00000000 255.255.255.0 Network.Network.Network.Host
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EXAMPLE Network 130.5.0.0 Class B (128-191, high order bits = 10) Without subnetting Network = 1 (for this IP Address) Hosts = 65,534 (216-2 or 65,536 -2) (one for the network address and one for the broadcast address) Class B Default Network Mask 11111111.11111111.00000000.00000000 255.255.0.0 Network.Network.Host.Host Network 130.5.0.0 100000010.00000101.00000000.00000000 Hosts (65,534) 130.5.0.1 100000010.00000101.00000000.00000001 130.5.0.2 100000010.00000101.00000000.00000010 to 130.5.255.254 100000010.00000101.11111111.11111110 Broadcast 130.5.255.255 100000010.00000101.11111111.11111111 With subnetting (just one possibility) Network IP Address 130.5.0.0 100000010.00000101.00000000.00000000 Let's borrow 8 bits from the Host-Number to form a Subnet-Number
Network-Prefix Host-Number
Network-Prefix Subnet-Number Host-Number
Extended-Network-Prefix
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Subnet Mask: 255.255.255.0
11111111 . 11111111 . 11111111 . 00000000
Extended-Network-Prefix
Remember: Class B Default Network Mask 255.255.0.0
11111111 . 11111111 . 00000000 . 00000000
Extended-Network-Prefix
Before we use the mask, lets look at what it does... 130.5.0.0
Base Network 10000010 . 00000101 . 00000000 . 00000000
255.255.255.0 or /24 Network and Subnet portion -> all 1's Host portion -> all 0's
Base Network 100000010 . 00000101 . 00000000 . 00000000
Network Portion . Subnet . Host
Extended-Network-Prefix
Network . Network . Subnet . Host
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Network Portion Network = 1 130.5.0.0 (The Base Net) Subnet Information Subnet Mask = 255.255.255.0 Or /24 Subnet Portion Subnets = 254 8 bits 254 (256-2) Subnets 00000000 to 11111111 (The -2 is for the network and broadcast addresses of the base net. Can't tell difference between which is for the network and which is for the router. - we'll see it soon) Host Portion Hosts = 254 8 bits 254 (256-2) Hosts 00000000 to 11111111 (The -2 is for the network and broadcast addresses for each subnet- we'll see it soon) What does this mean to the Network Administrator? Network Administrator has now divided the single Class B (/16) network with 65,534 hosts into 254 subnetworks (subnets) with 254 hosts per subnet. (256-2 for subnets and hosts in each subnet to be explained)
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IP Subnet Calculation & Design IP Class: B IP Address: 130.5.0.0 Mask Bits: 8 Subnet Mask: 255.255.255.0 Subnets: 254 +1 IP Major Net: 130.5.0.0 Host/Subnets: 254 Major Net Broadcast: 130.5.255.255
Subnets for fixed length Subnet masking
Subnet # Network Hosts Broadcast Address
0 130.5.0.0 130.5.0.1 to 130.5.0.254 130.5.0.255 1 130.5.1.0 130.5.1.1 to 130.5.1.254 130.5.1.255 2 130.5.2.0 130.5.2.1 to 130.5.2.254 130.5.2.255 3 130.5.3.0 130.5.3.1 to 130.5.3.254 130.5.3.255 4 130.5.4.0 130.5.4.1 to 130.5.4.254 130.5.4.255 5 130.5.5.0 130.5.5.1 to 130.5.5.254 130.5.5.255 6 130.5.6.0 130.5.6.1 to 130.5.6.254 130.5.6.255 7 130.5.7.0 130.5.7.1 to 130.5.7.254 130.5.7.255 8 130.5.8.0 130.5.8.1 to 130.5.8.254 130.5.8.255 9 130.5.9.0 130.5.9.1 to 130.5.9.254 130.5.9.255
10 130.5.10.0 130.5.10.1 to 130.5.10.254 130.5.10.255 11 130.5.11.0 130.5.11.1 to 130.5.11.254 130.5.11.255 12 130.5.12.0 130.5.12.1 to 130.5.12.254 130.5.12.255 13 130.5.13.0 130.5.13.1 to 130.5.13.254 130.5.13.255 14 130.5.14.0 130.5.14.1 to 130.5.14.254 130.5.14.255 15 130.5.15.0 130.5.15.1 to 130.5.15.254 130.5.15.255 16 130.5.16.0 130.5.16.1 to 130.5.16.254 130.5.16.255 17 130.5.17.0 130.5.17.1 to 130.5.17.254 130.5.17.255 18 130.5.18.0 130.5.18.1 to 130.5.18.254 130.5.18.255 19 130.5.19.0 130.5.19.1 to 130.5.19.254 130.5.19.255 20 130.5.20.0 130.5.20.1 to 130.5.20.254 130.5.20.255 21 130.5.21.0 130.5.21.1 to 130.5.21.254 130.5.21.255 22 130.5.22.0 130.5.22.1 to 130.5.22.254 130.5.22.255 23 130.5.23.0 130.5.23.1 to 130.5.23.254 130.5.23.255 24 130.5.24.0 130.5.24.1 to 130.5.24.254 130.5.24.255 25 130.5.25.0 130.5.25.1 to 130.5.25.254 130.5.25.255 26 130.5.26.0 130.5.26.1 to 130.5.26.254 130.5.26.255 27 130.5.27.0 130.5.27.1 to 130.5.27.254 130.5.27.255 28 130.5.28.0 130.5.28.1 to 130.5.28.254 130.5.28.255
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29 130.5.29.0 130.5.29.1 to 130.5.29.254 130.5.29.255 30 130.5.30.0 130.5.30.1 to 130.5.30.254 130.5.30.255 31 130.5.31.0 130.5.31.1 to 130.5.31.254 130.5.31.255 32 130.5.32.0 130.5.32.1 to 130.5.32.254 130.5.32.255 33 130.5.33.0 130.5.33.1 to 130.5.33.254 130.5.33.255 34 130.5.34.0 130.5.34.1 to 130.5.34.254 130.5.34.255 35 130.5.35.0 130.5.35.1 to 130.5.35.254 130.5.35.255 36 130.5.36.0 130.5.36.1 to 130.5.36.254 130.5.36.255 37 130.5.37.0 130.5.37.1 to 130.5.37.254 130.5.37.255 38 130.5.38.0 130.5.38.1 to 130.5.38.254 130.5.38.255 39 130.5.39.0 130.5.39.1 to 130.5.39.254 130.5.39.255 40 130.5.40.0 130.5.40.1 to 130.5.40.254 130.5.40.255 41 130.5.41.0 130.5.41.1 to 130.5.41.254 130.5.41.255 42 130.5.42.0 130.5.42.1 to 130.5.42.254 130.5.42.255 43 130.5.43.0 130.5.43.1 to 130.5.43.254 130.5.43.255 44 130.5.44.0 130.5.44.1 to 130.5.44.254 130.5.44.255 45 130.5.45.0 130.5.45.1 to 130.5.45.254 130.5.45.255 46 130.5.46.0 130.5.46.1 to 130.5.46.254 130.5.46.255 47 130.5.47.0 130.5.47.1 to 130.5.47.254 130.5.47.255 48 130.5.48.0 130.5.48.1 to 130.5.48.254 130.5.48.255 49 130.5.49.0 130.5.49.1 to 130.5.49.254 130.5.49.255 50 130.5.50.0 130.5.50.1 to 130.5.50.254 130.5.50.255 51 130.5.51.0 130.5.51.1 to 130.5.51.254 130.5.51.255 52 130.5.52.0 130.5.52.1 to 130.5.52.254 130.5.52.255 53 130.5.53.0 130.5.53.1 to 130.5.53.254 130.5.53.255 54 130.5.54.0 130.5.54.1 to 130.5.54.254 130.5.54.255 55 130.5.55.0 130.5.55.1 to 130.5.55.254 130.5.55.255 56 130.5.56.0 130.5.56.1 to 130.5.56.254 130.5.56.255 57 130.5.57.0 130.5.57.1 to 130.5.57.254 130.5.57.255 58 130.5.58.0 130.5.58.1 to 130.5.58.254 130.5.58.255 59 130.5.59.0 130.5.59.1 to 130.5.59.254 130.5.59.255 60 130.5.60.0 130.5.60.1 to 130.5.60.254 130.5.60.255 61 130.5.61.0 130.5.61.1 to 130.5.61.254 130.5.61.255 62 130.5.62.0 130.5.62.1 to 130.5.62.254 130.5.62.255 63 130.5.63.0 130.5.63.1 to 130.5.63.254 130.5.63.255 64 130.5.64.0 130.5.64.1 to 130.5.64.254 130.5.64.255 65 130.5.65.0 130.5.65.1 to 130.5.65.254 130.5.65.255 66 130.5.66.0 130.5.66.1 to 130.5.66.254 130.5.66.255 67 130.5.67.0 130.5.67.1 to 130.5.67.254 130.5.67.255 68 130.5.68.0 130.5.68.1 to 130.5.68.254 130.5.68.255
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69 130.5.69.0 130.5.69.1 to 130.5.69.254 130.5.69.255 70 130.5.70.0 130.5.70.1 to 130.5.70.254 130.5.70.255 71 130.5.71.0 130.5.71.1 to 130.5.71.254 130.5.71.255 72 130.5.72.0 130.5.72.1 to 130.5.72.254 130.5.72.255 73 130.5.73.0 130.5.73.1 to 130.5.73.254 130.5.73.255 74 130.5.74.0 130.5.74.1 to 130.5.74.254 130.5.74.255 75 130.5.75.0 130.5.75.1 to 130.5.75.254 130.5.75.255 76 130.5.76.0 130.5.76.1 to 130.5.76.254 130.5.76.255 77 130.5.77.0 130.5.77.1 to 130.5.77.254 130.5.77.255 78 130.5.78.0 130.5.78.1 to 130.5.78.254 130.5.78.255 79 130.5.79.0 130.5.79.1 to 130.5.79.254 130.5.79.255 80 130.5.80.0 130.5.80.1 to 130.5.80.254 130.5.80.255 81 130.5.81.0 130.5.81.1 to 130.5.81.254 130.5.81.255 82 130.5.82.0 130.5.82.1 to 130.5.82.254 130.5.82.255 83 130.5.83.0 130.5.83.1 to 130.5.83.254 130.5.83.255 84 130.5.84.0 130.5.84.1 to 130.5.84.254 130.5.84.255 85 130.5.85.0 130.5.85.1 to 130.5.85.254 130.5.85.255 86 130.5.86.0 130.5.86.1 to 130.5.86.254 130.5.86.255 87 130.5.87.0 130.5.87.1 to 130.5.87.254 130.5.87.255 88 130.5.88.0 130.5.88.1 to 130.5.88.254 130.5.88.255 89 130.5.89.0 130.5.89.1 to 130.5.89.254 130.5.89.255 90 130.5.90.0 130.5.90.1 to 130.5.90.254 130.5.90.255 91 130.5.91.0 130.5.91.1 to 130.5.91.254 130.5.91.255 92 130.5.92.0 130.5.92.1 to 130.5.92.254 130.5.92.255 93 130.5.93.0 130.5.93.1 to 130.5.93.254 130.5.93.255 94 130.5.94.0 130.5.94.1 to 130.5.94.254 130.5.94.255 95 130.5.95.0 130.5.95.1 to 130.5.95.254 130.5.95.255 96 130.5.96.0 130.5.96.1 to 130.5.96.254 130.5.96.255 97 130.5.97.0 130.5.97.1 to 130.5.97.254 130.5.97.255 98 130.5.98.0 130.5.98.1 to 130.5.98.254 130.5.98.255 99 130.5.99.0 130.5.99.1 to 130.5.99.254 130.5.99.255
100 130.5.100.0 130.5.100.1 to 130.5.100.254 130.5.100.255 101 130.5.101.0 130.5.101.1 to 130.5.101.254 130.5.101.255 102 130.5.102.0 130.5.102.1 to 130.5.102.254 130.5.102.255 103 130.5.103.0 130.5.103.1 to 130.5.103.254 130.5.103.255 104 130.5.104.0 130.5.104.1 to 130.5.104.254 130.5.104.255 105 130.5.105.0 130.5.105.1 to 130.5.105.254 130.5.105.255 106 130.5.106.0 130.5.106.1 to 130.5.106.254 130.5.106.255 107 130.5.107.0 130.5.107.1 to 130.5.107.254 130.5.107.255 108 130.5.108.0 130.5.108.1 to 130.5.108.254 130.5.108.255
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109 130.5.109.0 130.5.109.1 to 130.5.109.254 130.5.109.255 110 130.5.110.0 130.5.110.1 to 130.5.110.254 130.5.110.255 111 130.5.111.0 130.5.111.1 to 130.5.111.254 130.5.111.255 112 130.5.112.0 130.5.112.1 to 130.5.112.254 130.5.112.255 113 130.5.113.0 130.5.113.1 to 130.5.113.254 130.5.113.255 114 130.5.114.0 130.5.114.1 to 130.5.114.254 130.5.114.255 115 130.5.115.0 130.5.115.1 to 130.5.115.254 130.5.115.255 116 130.5.116.0 130.5.116.1 to 130.5.116.254 130.5.116.255 117 130.5.117.0 130.5.117.1 to 130.5.117.254 130.5.117.255 118 130.5.118.0 130.5.118.1 to 130.5.118.254 130.5.118.255 119 130.5.119.0 130.5.119.1 to 130.5.119.254 130.5.119.255 120 130.5.120.0 130.5.120.1 to 130.5.120.254 130.5.120.255 121 130.5.121.0 130.5.121.1 to 130.5.121.254 130.5.121.255 122 130.5.122.0 130.5.122.1 to 130.5.122.254 130.5.122.255 123 130.5.123.0 130.5.123.1 to 130.5.123.254 130.5.123.255 124 130.5.124.0 130.5.124.1 to 130.5.124.254 130.5.124.255 125 130.5.125.0 130.5.125.1 to 130.5.125.254 130.5.125.255 126 130.5.126.0 130.5.126.1 to 130.5.126.254 130.5.126.255 127 130.5.127.0 130.5.127.1 to 130.5.127.254 130.5.127.255 128 130.5.128.0 130.5.128.1 to 130.5.128.254 130.5.128.255 129 130.5.129.0 130.5.129.1 to 130.5.129.254 130.5.129.255 130 130.5.130.0 130.5.130.1 to 130.5.130.254 130.5.130.255 131 130.5.131.0 130.5.131.1 to 130.5.131.254 130.5.131.255 132 130.5.132.0 130.5.132.1 to 130.5.132.254 130.5.132.255 133 130.5.133.0 130.5.133.1 to 130.5.133.254 130.5.133.255 134 130.5.134.0 130.5.134.1 to 130.5.134.254 130.5.134.255 135 130.5.135.0 130.5.135.1 to 130.5.135.254 130.5.135.255 136 130.5.136.0 130.5.136.1 to 130.5.136.254 130.5.136.255 137 130.5.137.0 130.5.137.1 to 130.5.137.254 130.5.137.255 138 130.5.138.0 130.5.138.1 to 130.5.138.254 130.5.138.255 139 130.5.139.0 130.5.139.1 to 130.5.139.254 130.5.139.255 140 130.5.140.0 130.5.140.1 to 130.5.140.254 130.5.140.255 141 130.5.141.0 130.5.141.1 to 130.5.141.254 130.5.141.255 142 130.5.142.0 130.5.142.1 to 130.5.142.254 130.5.142.255 143 130.5.143.0 130.5.143.1 to 130.5.143.254 130.5.143.255 144 130.5.144.0 130.5.144.1 to 130.5.144.254 130.5.144.255 145 130.5.145.0 130.5.145.1 to 130.5.145.254 130.5.145.255 146 130.5.146.0 130.5.146.1 to 130.5.146.254 130.5.146.255 147 130.5.147.0 130.5.147.1 to 130.5.147.254 130.5.147.255 148 130.5.148.0 130.5.148.1 to 130.5.148.254 130.5.148.255
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149 130.5.149.0 130.5.149.1 to 130.5.149.254 130.5.149.255 150 130.5.150.0 130.5.150.1 to 130.5.150.254 130.5.150.255 151 130.5.151.0 130.5.151.1 to 130.5.151.254 130.5.151.255 152 130.5.152.0 130.5.152.1 to 130.5.152.254 130.5.152.255 153 130.5.153.0 130.5.153.1 to 130.5.153.254 130.5.153.255 154 130.5.154.0 130.5.154.1 to 130.5.154.254 130.5.154.255 155 130.5.155.0 130.5.155.1 to 130.5.155.254 130.5.155.255 156 130.5.156.0 130.5.156.1 to 130.5.156.254 130.5.156.255 157 130.5.157.0 130.5.157.1 to 130.5.157.254 130.5.157.255 158 130.5.158.0 130.5.158.1 to 130.5.158.254 130.5.158.255 159 130.5.159.0 130.5.159.1 to 130.5.159.254 130.5.159.255 160 130.5.160.0 130.5.160.1 to 130.5.160.254 130.5.160.255 161 130.5.161.0 130.5.161.1 to 130.5.161.254 130.5.161.255 162 130.5.162.0 130.5.162.1 to 130.5.162.254 130.5.162.255 163 130.5.163.0 130.5.163.1 to 130.5.163.254 130.5.163.255 164 130.5.164.0 130.5.164.1 to 130.5.164.254 130.5.164.255 165 130.5.165.0 130.5.165.1 to 130.5.165.254 130.5.165.255 166 130.5.166.0 130.5.166.1 to 130.5.166.254 130.5.166.255 167 130.5.167.0 130.5.167.1 to 130.5.167.254 130.5.167.255 168 130.5.168.0 130.5.168.1 to 130.5.168.254 130.5.168.255 169 130.5.169.0 130.5.169.1 to 130.5.169.254 130.5.169.255 170 130.5.170.0 130.5.170.1 to 130.5.170.254 130.5.170.255 171 130.5.171.0 130.5.171.1 to 130.5.171.254 130.5.171.255 172 130.5.172.0 130.5.172.1 to 130.5.172.254 130.5.172.255 173 130.5.173.0 130.5.173.1 to 130.5.173.254 130.5.173.255 174 130.5.174.0 130.5.174.1 to 130.5.174.254 130.5.174.255 175 130.5.175.0 130.5.175.1 to 130.5.175.254 130.5.175.255 176 130.5.176.0 130.5.176.1 to 130.5.176.254 130.5.176.255 177 130.5.177.0 130.5.177.1 to 130.5.177.254 130.5.177.255 178 130.5.178.0 130.5.178.1 to 130.5.178.254 130.5.178.255 179 130.5.179.0 130.5.179.1 to 130.5.179.254 130.5.179.255 180 130.5.180.0 130.5.180.1 to 130.5.180.254 130.5.180.255 181 130.5.181.0 130.5.181.1 to 130.5.181.254 130.5.181.255 182 130.5.182.0 130.5.182.1 to 130.5.182.254 130.5.182.255 183 130.5.183.0 130.5.183.1 to 130.5.183.254 130.5.183.255 184 130.5.184.0 130.5.184.1 to 130.5.184.254 130.5.184.255 185 130.5.185.0 130.5.185.1 to 130.5.185.254 130.5.185.255 186 130.5.186.0 130.5.186.1 to 130.5.186.254 130.5.186.255 187 130.5.187.0 130.5.187.1 to 130.5.187.254 130.5.187.255 188 130.5.188.0 130.5.188.1 to 130.5.188.254 130.5.188.255
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189 130.5.189.0 130.5.189.1 to 130.5.189.254 130.5.189.255 190 130.5.190.0 130.5.190.1 to 130.5.190.254 130.5.190.255 191 130.5.191.0 130.5.191.1 to 130.5.191.254 130.5.191.255 192 130.5.192.0 130.5.192.1 to 130.5.192.254 130.5.192.255 193 130.5.193.0 130.5.193.1 to 130.5.193.254 130.5.193.255 194 130.5.194.0 130.5.194.1 to 130.5.194.254 130.5.194.255 195 130.5.195.0 130.5.195.1 to 130.5.195.254 130.5.195.255 196 130.5.196.0 130.5.196.1 to 130.5.196.254 130.5.196.255 197 130.5.197.0 130.5.197.1 to 130.5.197.254 130.5.197.255 198 130.5.198.0 130.5.198.1 to 130.5.198.254 130.5.198.255 199 130.5.199.0 130.5.199.1 to 130.5.199.254 130.5.199.255 200 130.5.200.0 130.5.200.1 to 130.5.200.254 130.5.200.255 201 130.5.201.0 130.5.201.1 to 130.5.201.254 130.5.201.255 202 130.5.202.0 130.5.202.1 to 130.5.202.254 130.5.202.255 203 130.5.203.0 130.5.203.1 to 130.5.203.254 130.5.203.255 204 130.5.204.0 130.5.204.1 to 130.5.204.254 130.5.204.255 205 130.5.205.0 130.5.205.1 to 130.5.205.254 130.5.205.255 206 130.5.206.0 130.5.206.1 to 130.5.206.254 130.5.206.255 207 130.5.207.0 130.5.207.1 to 130.5.207.254 130.5.207.255 208 130.5.208.0 130.5.208.1 to 130.5.208.254 130.5.208.255 209 130.5.209.0 130.5.209.1 to 130.5.209.254 130.5.209.255 210 130.5.210.0 130.5.210.1 to 130.5.210.254 130.5.210.255 211 130.5.211.0 130.5.211.1 to 130.5.211.254 130.5.211.255 212 130.5.212.0 130.5.212.1 to 130.5.212.254 130.5.212.255 213 130.5.213.0 130.5.213.1 to 130.5.213.254 130.5.213.255 214 130.5.214.0 130.5.214.1 to 130.5.214.254 130.5.214.255 215 130.5.215.0 130.5.215.1 to 130.5.215.254 130.5.215.255 216 130.5.216.0 130.5.216.1 to 130.5.216.254 130.5.216.255 217 130.5.217.0 130.5.217.1 to 130.5.217.254 130.5.217.255 218 130.5.218.0 130.5.218.1 to 130.5.218.254 130.5.218.255 219 130.5.219.0 130.5.219.1 to 130.5.219.254 130.5.219.255 220 130.5.220.0 130.5.220.1 to 130.5.220.254 130.5.220.255 221 130.5.221.0 130.5.221.1 to 130.5.221.254 130.5.221.255 222 130.5.222.0 130.5.222.1 to 130.5.222.254 130.5.222.255 223 130.5.223.0 130.5.223.1 to 130.5.223.254 130.5.223.255 224 130.5.224.0 130.5.224.1 to 130.5.224.254 130.5.224.255 225 130.5.225.0 130.5.225.1 to 130.5.225.254 130.5.225.255 226 130.5.226.0 130.5.226.1 to 130.5.226.254 130.5.226.255 227 130.5.227.0 130.5.227.1 to 130.5.227.254 130.5.227.255 228 130.5.228.0 130.5.228.1 to 130.5.228.254 130.5.228.255
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229 130.5.229.0 130.5.229.1 to 130.5.229.254 130.5.229.255 230 130.5.230.0 130.5.230.1 to 130.5.230.254 130.5.230.255 231 130.5.231.0 130.5.231.1 to 130.5.231.254 130.5.231.255 232 130.5.232.0 130.5.232.1 to 130.5.232.254 130.5.232.255 233 130.5.233.0 130.5.233.1 to 130.5.233.254 130.5.233.255 234 130.5.234.0 130.5.234.1 to 130.5.234.254 130.5.234.255 235 130.5.235.0 130.5.235.1 to 130.5.235.254 130.5.235.255 236 130.5.236.0 130.5.236.1 to 130.5.236.254 130.5.236.255 237 130.5.237.0 130.5.237.1 to 130.5.237.254 130.5.237.255 238 130.5.238.0 130.5.238.1 to 130.5.238.254 130.5.238.255 239 130.5.239.0 130.5.239.1 to 130.5.239.254 130.5.239.255 240 130.5.240.0 130.5.240.1 to 130.5.240.254 130.5.240.255 241 130.5.241.0 130.5.241.1 to 130.5.241.254 130.5.241.255 242 130.5.242.0 130.5.242.1 to 130.5.242.254 130.5.242.255 243 130.5.243.0 130.5.243.1 to 130.5.243.254 130.5.243.255 244 130.5.244.0 130.5.244.1 to 130.5.244.254 130.5.244.255 245 130.5.245.0 130.5.245.1 to 130.5.245.254 130.5.245.255 246 130.5.246.0 130.5.246.1 to 130.5.246.254 130.5.246.255 247 130.5.247.0 130.5.247.1 to 130.5.247.254 130.5.247.255 248 130.5.248.0 130.5.248.1 to 130.5.248.254 130.5.248.255 249 130.5.249.0 130.5.249.1 to 130.5.249.254 130.5.249.255 250 130.5.250.0 130.5.250.1 to 130.5.250.254 130.5.250.255 251 130.5.251.0 130.5.251.1 to 130.5.251.254 130.5.251.255 252 130.5.252.0 130.5.252.1 to 130.5.252.254 130.5.252.255 253 130.5.253.0 130.5.253.1 to 130.5.253.254 130.5.253.255 254 130.5.254.0 130.5.254.1 to 130.5.254.254 130.5.254.255 255 130.5.255.0 130.5.255.1 to 130.5.255.254 130.5.255.255
Don’t use subnet 0 (unless using ip subnet-zero command) and subnet 255. Address space wasted by subnetting = 1.55% (1.17% with ip subnet-zero)
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The Base Net Address or the Address for the entire 130.5.0.0 Network
Base Network 10000010 . 00000101 . 00000000 . 00000000
First "Usable" Subnet ... Subnet Address 130.5.1.0 10000010.00000101.00000001.00000000 Hosts (254) 130.5.1.1 10000010.00000101.00000001.00000001 130.5.1.2 10000010.00000101.00000001.00000010 130.5.1.3 10000010.00000101.00000001.00000011 130.5.1.4 10000010.00000101.00000001.00000100 to 130.5.1.254 .10000010.00000101.00000001.11111110 Broadcast for Subnet 130.5.1.0 130.5.1.255 10000010.00000101.00000001.11111111 Next Subnet … Subnet Address 130-5.2.0 10000010.00000101.00000010.00000000 Hosts (254) 130.5.2.1 10000010.00000101.00000010.00000001 130.5.2.2 10000010.00000101.00000010.00000010 130.5.2.3 10000010.00000101.00000010.00000011 130.5.2.4 10000010.00000101.00000010.00000100 to 130.5.2.254 10000010.00000101.00000010.11111110
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Broadcast for Subnet 130.5.2.0 130.5.2.255 .10000010.00000101.00000010.11111111 To the last Subnet … Subnet Address 130.5.254.0 10000010.00000101.11111110.00000000 Hosts (254) 130.5.254.1 10000010.00000101.11111110.00000001 130.5.254.2 10000010.00000101.11111110.00000010 130.5.254.3 10000010.00000101.11111110.00000011 130.5.254.4 10000010.00000101.11111110.00000100 to 130.5.254.254 10000010.00000101.11111110.11111110 Broadcast for Subnet 130.5.254.0 130.5.254.255 10000010.00000101.11111110.11111111 The Broadcast Address for Base Net, the entire 130.5.0.0 Network 130.5.255.255 10000010.00000101.11111111.11111111
Why not the Network 130.5.0.0? 130.5.0.0 10000010.00000101.00000000.00000000 (Which would have the hosts 130.5.0.1 through 130.5.0.254, and broadcast address 130.5.0.255) Because 130.5.0.0 10000010.00000101.00000000.00000000 Is this the Network address for just this Subnet, 130.5.0.0, or is the Network address for the entire 130.5.0.0 Network? We don't know, so the router doesn't know. This Subnet not used!
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Why not the Network 130.5.255.0? 130.5.255.0 10000010.00000101.11111111.00000000 Because 130.5.255.255 10000010.00000101.11111111.11111111 Is this the broadcast address for just this Subnet, 130.5.255.0, or is the broadcast address for the entire 130.5-0.0 Network? We don't know, so the router doesn't know. This Subnet not used! NOTE: 0 Zero subnets are possible with some routers, but are not recommended. What else are we losing besides all of the hosts in the first and last subnets ? Can we use the first IP Address of a subnet for a host address?
No, because that is the IP Address for that subnet. Can we use last IP Address of a subnet for a host address?
No, because that is the broadcast address for that subnet. Making the Box Using the previous IP Subnet Calculator example, we can make a box to cross out all of the unusable IP Addresses for hosts. This shows us what we lose when we subnet.
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Extended-Network-Prefix versus Subnet Mask Modem routing protocols (RIP-2, OSPF, - coming) and standards (RFCs, journals, etc.) more
often refer to the "Extended-Network-Prefix" rather than the "subnet mask." The Extended-Network-Prefix is equal to the number of contiguous one-bits in the traditional
subnet mask. The Extended-Network-Prefix is expressed using "/" (slash) followed by the number of
contiguous one-bits Format Network address: 130.5.5.25 Subnet mask: 255.255.255.0 (11111111.11111111.11111111.00000000) Can also be expressed as: 130.5.5.25/24 /24 same as 255.255.255.0 Easier and more compact notation We will see in later discussions... However, modem routing protocols still must carry the subnet mask (as we will see later). There are no Internet standards for a one-byte field in the routing protocol headers. Each routing protocol is still required to carry the complete four-octet traditional subnet
mask.
How Subnet Masks are Used by an "Interior" Router Reminder... Subnet Mask separates Network/Subnet address from Host address Subnet Mask (Cisco) = 32 bit address mask used in IP to indicate the bits of an IP address that are being used for the subnet address. Sometimes referred to simply as a mask.
Subnet Mask 32 bits long Divided into four octets Network and Subnet portion -> all 1's Host portion -> all 0's
When a router examines an IP address it sees it as a binary address, not in dotted decimal address.
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The router takes the IP address being examined and the subnet mask and logically ANDs them together. This will result in discovering the network/subnetwork number. AND operation 1 AND 1 equals 1 1 AND 0 equals 0 0 AND 0 equals 0
Destination IP Address: 130.5.5.25
Network Portion . Subnet . Host
IP Address: 130.5.5.25 10000010 . 00000101 . 00000101 . 00011001
AND
Subnet: /24 255.255.255.0
11111111 . 11111111 . 11111111 . 00000000
10000010 . 00000101 . 00000101 . 00000000
Private networks
Gateway RouterDA = 130.5.5.25 Router
191.11.0.2Has Subnet MaskPerforms ANDOperation
130.5.1.0
130.5.2.0
130.5.3.0
etc.
130.5.0.0 via190.11.0.2No subnet info
Exterior routers don't have view of subnets, only of the whole network (the classful address
or base net address) Only Gateway and interior routers have subnet information, view of internal subnet structure. Router performs AND operation using IP address and subnet mask This tells router to route the packet, 130.5.5.25 to subnet 130.5.5.0
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How about the destination IP Address 130.5.65.1?
Network Portion . Subnet . Host
IP Address: 130.5.65.1 . . .
AND
Subnet: /24 255.255.255.0
. . .
. . .
Private
Gateway RouterDA = 130.5.65.1 Router
191.11.0.Has SubnetPerformsOperatio
130.5.1.0
130.5.2.0
130.5.64.0
etc.
130.5.0.0190.11.0.No subnet
130.5.65.0
etc.
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How about the destination IP Address 130.5.65.1? Answer
Network Portion . Subnet . Host
IP Address: 130.5.65.1 10000010 . 00000101 . 01000001 . 00000001
AND
Subnet: /24 255.255.255.0
11111111 . 11111111 . 11111111 . 00000000
10000010 . 00000101 . 01000001 . 00000000
Private networks
Gateway RouterDA = 130.5.65.1 Router
191.11.0.2Has Subnet MaskPerforms ANDOperation
130.5.1.0
130.5.2.0
130.5.64.0
etc.
130.5.0.0 via190.11.0.2No subnet info
130.5.65.0
etc. Once again Exterior routers don't have view of subnets, only of the whole network (the classful address
or base net address) Only Gateway and interior routers have subnet information, view of internal subnet structure. Router performs AND operation using IP address and subnet mask This tells router to route
the packet, 130.5.65.1, to subnet 130.5.65.0
Natural and other Boundaries This has been of a Class B network address, subnetted on a natural boundary. Natural boundary = Subnets and Hosts are separated at an dotted decimal, octet boundary
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Here is a Class B subnet divided on a natural boundary:
IP Address: 130.5.0.0 10000010 . 00000101 . 00000000 . 00000000
Q: Can we divide this subnet in other combinations of subnetworks and hosts? A: Yes!
Adding Bits to the Default Subnet Mask We learn that each class of address has a default mask. If we want to subnet a network, we add some bits to the default subnet mask, which reduces the number of bits used for the host address. The number of bits we add to the default mask determines the number of subnets wee can configure.
Subnetting Options and Design Considerations 1) How many subnets does your organization need, today and in the future? 2) How many hosts will you need on the largest subnet?
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Before we show the different combinations for a Class B(/24) network, let's look at simpler Class C Network. Class C (/24 prefixes) (192.0.0.xxx through 223.255.255.255) You have been given the following Base Net IP Address: • Base Net: 194.1.1.0 Without subnetting your only option is: • Networks = 1 • Hosts = 254 (256 - 2)
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Class C Example Base Net: 194.1.1.0/24 Subnet Mask: 255.255.255.240/28
IP Address: 130.5.0.0 10000010 . 00000001 . 00000001 . 00000000
Subnets: 14 (16-2); -2, network address and broadcast address for base net 0000 to 1111 Hosts per subnet: 14 (16-2); -2, network address and broadcast address for the subnet
List of Subnets for the 194.1.1.0 network with the subnet mask 255.255.255.240
Subnet # Network Hosts Binary Representation
1 194.1.1.0 194.1.1.1 to 194.1.1.14 11000010.00000001.00000001.00000000 2 194.1.1.16 194.1.1.17 to 194.1.1.30 11000010.00000001.00000001.00010000 3 194.1.1.32 194.1.1.33 to 194.1.1.46 11000010.00000001.00000001.00100000 4 194.1.1.48 194.1.1.49 to 194.1.1.62 11000010.00000001.00000001.00110000 5 194.1.1.64 194.1.1.65 to 194.1.1.78 11000010.00000001.00000001.01000000 6 194.1.1.80 194.1.1.81 to 194.1.1.94 11000010.00000001.00000001.01010000 7 194.1.1.96 194.1.1.97 to 194.1.1.110 11000010.00000001.00000001.01100000 8 194.1.1.112 194.1.1.113 to 194.1.1.126 11000010.00000001.00000001.01110000 9 194.1.1.128 194.1.1.129 to 194.1.1.142 11000010.00000001.00000001.10000000 10 194.1.1.144 194.1.1.145 to 194.1.1.158 11000010.00000001.00000001.10010000 11 194.1.1.160 194.1.1.161 to 194.1.1.174 11000010.00000001.00000001.10100000 12 194.1.1.176 194.1.1.177 to 194.1.1.190 11000010.00000001.00000001.10110000 13 194.1.1.192 194.1.1.193 to 194.1.1.206 11000010.00000001.00000001.11000000 14 194.1.1.208 194.1.1.209 to 194.1.1.222 11000010.00000001.00000001.11010000 15 194.1.1.224 194.1.1.225 to 194.1.1.238 11000010.00000001.00000001.11100000 16 194.1.1.240 194.1.1.241 to 194.1.1.254 11000010.00000001.00000001.11110000
Back to Subnet #1 and Subnet #16 which cannot be used. Subnet #1: Subnet Add.: 194.1.1.16/24 Host #1: 191.1.1.1/24 11000010.00000001.00000001.00000001 Host #2: 191.1.1.2/24 11000010.00000001.00000001.00000010 Host #3: 191.1.1.3/24 11000010.00000001.00000001.00000011 Host #4: 191.1.1.4/24 11000010.00000001.00000001.00000100 Host #5: 191.1.1.5/24 11000010.00000001.00000001.00000101 Host #6: 191.1.1.6/24 11000010.00000001.00000001.00000110 Host #7: 191.1.1.7/24 11000010.00000001.00000001.00000111 Host #8: 191.1.1.8/24 11000010.00000001.00000001.00001000 Host #9: 191.1.1.9/24 11000010.00000001.00000001.00001001 Host # 10: 191.1.1.10/24 11000010.00000001.00000001.00001010 Host #11: 191.1.1.11/24 11000010.00000001.00000001.00001011 Host # 12: 191.1.1.12/24 11000010.00000001.00000001.00001100 Host # 13: 191.1.1.13/24 11000010.00000001.00000001.00001101 Host #14: 191.1.1.14/24 11000010.00000001.00000001.00001110 Broadcast: 191.1.1.15/24 11000010.00000001.00000001.00001111 What is the network address for Subnet #1? 194.1.1.0/24 11000010.00000001.00000001.00000000 What is the network address for the Base Net (for all of the subnets)? 194.1.1.0/24 11000010.00000001.00000001.00000000 Same Address! • Is this the network address for Subnet #1 or is this the network address for the entire base
net? We can't tell and routers (usually) can't tell either. • So this subnet not used, which also wastes 14 host addresses.
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Let's take a look at what Subnet #16 would look like: Subnet #16: Subnet Add.: 194.1.1.240/24 Host #1: 191.1.1.241/24 11000010.00000001.00000001.11110001 Host #2: 191.1.1.242/24 11000010.00000001.00000001.11110010 Host #3: 191.1.1.243/24 11000010.00000001.00000001.11110011 Host #4: 191.1.1.244/24 11000010.00000001.00000001.11110100 Host #5: 191.1.1.245/24 11000010.00000001.00000001.11110101 Host #6: 191.1.1.246/24 11000010.00000001.00000001.11110110 Host #7: 191.1.1.247/24 11000010.00000001.00000001.11110111 Host #8: 191.1.1.248/24 11000010.00000001.00000001.11111000 Host #9: 191.1.1.249/24 11000010.00000001.00000001.11111001 Host # 10: 191.1.1.250/24 11000010.00000001.00000001.11111010 Host #11: 191.1.1.251/24 11000010.00000001.00000001.11111011 Host # 12: 191.1.1.252/24 11000010.00000001.00000001.11111100 Host # 13: 191.1.1.253/24 11000010.00000001.00000001.11111101 Host #14: 191.1.1.254/24 11000010.00000001.00000001.11111110 Broadcast: 191.1.1.255/24 11000010.00000001.00000001.00001111 What is the broadcast address for Subnet #16? 194.1.1.240/24 11000010.00000001.00000001.11111111 What is the broadcast address for the Base Net (for all of the subnets)? 194.1.1.0/24 11000010.00000001.00000001.11111111 Same Address! • Is this the broadcast address for Subnet #16 or is this the broadcast address for the entire base
net? We can't tell and routers (usually) can't tell either. • So this subnet not used, which also wastes 14 host addresses. Reasoning • RFC 950 (early days of subnetting) prohibitted the use of the all-0's and the all-1's subnets
This would eliminate situations which could cause confusion in a classful router. Today's routers can be both a Classful and Classless (later) router.
• Router could be running both RIP- I (classful routing protocol) and BGP-4 (classless protocol) at the same time.
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Subnet Calculator Results IP Subnet Calculation & Design IP Class: B IP Address: 130.5.0.0 Mask Bits: 4 Subnet Mask: 255.255.240.0 Subnets: 14+1 IP Major Net: 130.5.0.0 Hosts/Subnet: 4094 Major Net Bcast: 130.5.255.255 Subnets for Fixed Lenght Subnet Masking Subnet Subnet
Don't use subnet 0 (unless using ip subnet-zero command) and subnet 15. Address space wasted by subnetting = 12.54% ( 6.29% with ip subnet-zero) Notice: This chart above was taken from Cisco's Subnet Calculator, which begins the first subnet at Subnet 0. Making the Box: Which addresses can you NOT use for Hosts?
1) Cross out the first Row (Subnet 0): Subnet 0 address same as Base Net Address 2) Cross out the last Row (Subnet 15): Subnet 15 broadcast address same as Base Net
broadcast address. 3) Cross out the first Column (Subnet Address): Address of that subnet 4) Cross out the last Column (Broadcast Address): Broadcast address for that subnet
Bold -> Usable Hosts
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Subnetting Options and Design Considerations (once again) 1. How many subnets. does your organization need, today and in the future? 2. How many hosts will you need on the largest subnet?
