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

CIT 384: Network Administration Slide #1

CIT 384: Network Administration

Subnetting

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CIT 384: Network Administration Slide #2

Topics

1. IP Addresses

2. Classful and classless addressing

3. Subnet Masks and Prefixes

4. Subnet Math

5. Subnet Problems

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IP Addresses

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

172 . 16 . 254 . 1

10101100 00010000 11111110 00000001

1 byte

32 bits = 4 bytes

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Grouping IP Addresses

Groups of consecutive IP addrs are called networks.

Routing table would only need 3 entries below.

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Network and Host Parts

IP addresses are divided into two parts– Network ID (like zip code)– Host ID (like street address)

Network ID Host ID

Two special IP addresses– Network address (e.g. 130.4.0.0)– Broadcast address (e.g. 130.4.255.255)

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Address ClassesClass A: 0.0.0.0-127.255.255.255

8-bit net ID, 24-bit host ID224 – 2 hosts per network; 126 networks

Class B: 128.0.0.0-191.255.255.25516-bit net ID, 16-bit host ID216 – 2 hosts per network; 16,384 networks

Class 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 networks

Class D: 224.0.0.0-239.255.255.25528-bit multicast group ID

Class E: 240.0.0.0-255.255.255.255Reserved for future use

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CIDR

Classless Inter-Domain Routing– Classful 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

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Public IP Addresses

ICANN 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

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Private RFC1918 IP Addresses

Private IP Networks Network Class Count of Networks

10.0.0.0 A 1

172.16.0.0 through

172.31.0.0

B 16

192.168.0.0 through 192.168.255.0

C 256

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IPv4 vs IPv6 Addresses

Feature IPv4 IPv6

Size of Address 32 bits 128 bits

Example Address 10.1.1.1 0000:0000:0000:0000:FFFF:FFFF:0A01:0101

Abbreviated Address

- ::FFFF:FFFF:0A01:0101

Localhost 127.0.0.1 ::1/128

Possible Addresses

232 (~4 billion) 2128 (~3.4 x 1038)

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Network Mask

How do we list subnets in routing table?– Ex: addresses 150.150.4.0 – 150.150.4.255

– Table: 155.155.4.0 netmask 255.255.255.0

Subnet mask indicates range– Binary 1s indicate network part of address.

– Binary 0s indicate host part of address.

– Always consists of 1s followed by 0s.

Prefix notation– Humanly readable form of subnet mask.

– Just counts the number of binary 1s in mask.

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Classful Address Ranges and Masks

Class ANNNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class BNNNNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

Class CNNNNNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

Class Leading Bits

Start End Subnet Mask CIDR

A 0 0.0.0.0 126.255.255.255 255.0.0.0 /8

B 10 128.0.0.0 191.255.255.255 255.255.0.0 /16

C 110 192.0.0.0 231.255.255.255 255.255.255.0 /24

D 1110 224.0.0.0 239.255.255.255 N/A N/A

E 1111 240.0.0.0 255.255.255.0 N/A N/A

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Example

• IP Address: 137.201.18.42

• Address Class: B since 128 < 137 < 191

• Default Netmask: 255.255.0.0

• Network Address Part: 137.201.0.0

• Host Address Part: 0.0.18.42

• Broadcast Address: 137.201.255.255

• Host Address Range for Network:– 137.201.18.1 through 137.201.255.254

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Why Subnet?

Allows admin to create more networks for:1. Address conservation.

2. Organization of hosts.

3. Different physical media.

4. Security.

5. Performance (smaller broadcast domains.)

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IP Addresses with Subnets

Route on network + subnet part of address.

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Subnet Math

Binary <-> Decimal Conversion– Convert each byte of dotted quad into binary.

– Convert binary byte into 4 decimal values.

Boolean AND operation– 0 AND 0 = 0

– 0 AND 1 = 0

– 1 AND 0 = 0

– 1 AND 1 = 1

Convert between dotted quad and prefix.– 255.255.255.0 netmask is identical to /24

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How to find network address?

(IP Address) AND (Subnet Mask)

137.201.18.42 10001001.11001001.00010010.00101010

AND

255.255.0.0 11111111.11111111.00000000.00000000

---------------------------------------------------

10001001.11001001.00000000.00000000

(convert from binary to decimal)

137.201.0.0

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How to find number of networks?

Address divided between network and host.– If there are s subnet bits and h host bits, then

– Number of subnets = 2s

– Number of hosts = 2h – 2

Subnet zero– Classful routing reserves 2 subnets so only have 2s – 2.

• Lowest and highest subnet numbers.

– For Class B network 150.150.0.0 reserves• 150.150.0.0 (ambiguity with address of whole B)

• 150.150.255.0/24 (ambiguous broadcast 150.150.255.255)

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Choosing Subnet Mask to meet Design Requirements

Requirements– Class B network 130.1.0.0– Number of subnets: 200– Max hosts per subnet: 200

Problem 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.HHHHHHHH

Problem 3: find subnet mask– 11111111 11111111 11111111 00000000– 255.255.255.0

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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.HHHHHHHH– How 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?

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References

1. James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.

2. Cisco, Cisco Connection Documentation, http://www.cisco.com/univercd/home/home.htm

3. Cisco, Internetworking Basics, http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm

4. Matthew Gast, 802.11 Wireless Networks: The Definitive Guide, O’Reilly, 2005.

5. Wendell Odom, CCNA Official Exam Certification Library, 3rd edition, Cisco Press, 2007.


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