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CIT 384: Network AdministrationSlide #1 CIT 384: Network Administration Subnetting

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  • CIT 384: Network AdministrationSubnetting

    Computer Security: Art and Science

  • TopicsIP AddressesClassful and classless addressingSubnet Masks and PrefixesSubnet MathSubnet Problems

    Computer Security: Art and Science

  • IP Addresses32-bit integersOne for each network interface.Dotted decimal notation: ii.jj.kk.ll

    Computer Security: Art and Science

  • Grouping IP AddressesGroups of consecutive IP addrs are called networks.Routing table would only need 3 entries below.

    Computer Security: Art and Science

  • Network and Host PartsIP addresses are divided into two partsNetwork ID (like zip code)Host ID (like street address)Two special IP addressesNetwork address (e.g. 130.4.0.0)Broadcast address (e.g. 130.4.255.255)

    Computer Security: Art and Science

  • Address ClassesClass A: 0.0.0.0-127.255.255.2558-bit net ID, 24-bit host ID224 2 hosts per network; 126 networksClass B: 128.0.0.0-191.255.255.25516-bit net ID, 16-bit host ID216 2 hosts per network; 16,384 networksClass C: 192.0.0.0-223.255.255.25524-bit net ID, 8-bit host ID(28 2) = 254 hosts per network; 2,097,152 networksClass D: 224.0.0.0-239.255.255.25528-bit multicast group IDClass E: 240.0.0.0-255.255.255.255Reserved for future use

    Computer Security: Art and Science

  • CIDRClassless Inter-Domain RoutingClassful routing wastes most IP addresses.Allocate addresses on bit boundaries instead of byte boundaries.Allow ISPs/users to decide on boundaries instead of basing on IP addresses.Prefix notation/x indicates that first x bits are shared.192.168.0.0/16 = 192.168.0.0 192.168.255.255

    Computer Security: Art and Science

  • Public IP AddressesICANN assigns network numbers.Internet Corporation for Assigned Network Numbers.ICANN gives authority to regional orgs, e.g. ARIN (American Registry for Internet Numbers)Typically to ISPs, universities, corporations.ISP assigns IP addresses within network

    Computer Security: Art and Science

  • Private RFC1918 IP Addresses

    Computer Security: Art and Science

  • IPv4 vs IPv6 Addresses

    Computer Security: Art and Science

  • Network MaskHow do we list subnets in routing table?Ex: addresses 150.150.4.0 150.150.4.255Table: 155.155.4.0 netmask 255.255.255.0Subnet mask indicates rangeBinary 1s indicate network part of address.Binary 0s indicate host part of address.Always consists of 1s followed by 0s.Prefix notationHumanly readable form of subnet mask.Just counts the number of binary 1s in mask.

    Computer Security: Art and Science

  • Classful Address Ranges and MasksClass ANNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHHClass BNNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHHClass CNNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

    Computer Security: Art and Science

  • ExampleIP Address: 137.201.18.42Address Class: B since 128 < 137 < 191Default Netmask: 255.255.0.0Network Address Part: 137.201.0.0Host Address Part: 0.0.18.42Broadcast Address: 137.201.255.255Host Address Range for Network:137.201.18.1 through 137.201.255.254

    Computer Security: Art and Science

  • Why Subnet?Allows admin to create more networks for:Address conservation.Organization of hosts.Different physical media.Security.Performance (smaller broadcast domains.)

    Computer Security: Art and Science

  • IP Addresses with SubnetsRoute on network + subnet part of address.

    Computer Security: Art and Science

  • Subnet MathBinary Decimal ConversionConvert each byte of dotted quad into binary.Convert binary byte into 4 decimal values.Boolean AND operation0 AND 0 = 00 AND 1 = 01 AND 0 = 01 AND 1 = 1Convert between dotted quad and prefix.255.255.255.0 netmask is identical to /24

    Computer Security: Art and Science

  • How to find network address?(IP Address) AND (Subnet Mask)

    137.201.18.42 10001001.11001001.00010010.00101010AND 255.255.0.0 11111111.11111111.00000000.00000000 --------------------------------------------------- 10001001.11001001.00000000.00000000 (convert from binary to decimal) 137.201.0.0

    Computer Security: Art and Science

  • How to find number of networks?Address divided between network and host.If there are s subnet bits and h host bits, thenNumber of subnets = 2sNumber of hosts = 2h 2Subnet zeroClassful routing reserves 2 subnets so only have 2s 2.Lowest and highest subnet numbers.For Class B network 150.150.0.0 reserves150.150.0.0 (ambiguity with address of whole B)150.150.255.0/24 (ambiguous broadcast 150.150.255.255)

    Computer Security: Art and Science

  • Choosing Subnet Mask to meet Design RequirementsRequirementsClass B network 130.1.0.0Number of subnets: 200Max hosts per subnet: 200Problem 1: how many host bits?Find h, # of host bits, such that 2h >= 200.27 = 128, 28 = 256, therefore h = 8.Problem 2: how many subnet bits?Find s, # of subnet bits, such that 2s >= 200, yields s = 8.NNNNNNNN.NNNNNNNN.SSSSSSSS.HHHHHHHHProblem 3: find subnet mask11111111 11111111 11111111 00000000255.255.255.0

    Computer Security: Art and Science

  • Multiple Possible Subnet MasksIn some problems, many subnet masks exist.Ex: change # of subnets from 200 to 50.Find s, # of subnet bits, such that 2s >= 50, yields s = 6.NNNNNNNN.NNNNNNNN.SSSSSSxx.HHHHHHHHHow many possible subnets exist? 11111111 11111111 11111100 (6 subnet bits, 10 host bits)11111111 11111111 11111110 (7 subnet bits, 9 host bits)11111111 11111111 11111111 (8 subnet bits, 8 host bits)11111111 11111111 11111101 (impossible)Subnet masks/22 255.255.252.0 (6 subnet bits, 10 host bits, 1022 hosts/sub)/23 255.255.254.0 (7 subnet bits, 9 host bits, 510 hosts/sub)/24 255.255.255.0 (8 subnet bits, 8 host bits, 254 hosts/sub)Do you want to maximize # subnets or # hosts/subnet?

    Computer Security: Art and Science

  • ReferencesJames Boney, Cisco IOS in a Nutshell, 2nd edition, OReilly, 2005. Cisco, Cisco Connection Documentation, http://www.cisco.com/univercd/home/home.htmCisco, Internetworking Basics, http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htmMatthew Gast, 802.11 Wireless Networks: The Definitive Guide, OReilly, 2005.Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.

    Computer Security: Art and Science

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