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IP Addressing and Routing

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Basic IP Addressing

• Each host connected to the Internet is identified by a unique IP address.

• An IP address is a 32-bit quantity.Expressed as a dotted-decimal notation W.X.Y.Z.Consists of two logical parts:

A network numberA host number

This partition defines the IP address classes.

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IP Address Classes

• There are five defined IP address classes.

Class A UNICASTClass B UNICASTClass C UNICASTClass D MULTICASTClass E RESERVED

• There are some special-purpose IP addresses also.

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1111240.0.0.0 –255.255.255.255

E

1110224.0.0.0 –239.255.255.255

D

821110192.0.0.0 –223.255.255.255

C

161410128.0.0.0 –191.255.255.255

B

24700.0.0.0 –127.255.255.255

A

Host bits

Network bits

High-order bits

Address Range

Class

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Special-Purpose IP Addresses

Limited broadcast255.255.255.255

Reserved for private use192.168.0.0 –192.168.255.255

Reserved for private use172.16.0.0 –172.31.255.255

Reserved for loopback / local address

127.0.0.0 –127.255.255.255

Reserved for private use10.0.0.0 –10.255.255.255

Unknown network, commonly represents default

0.0.0.0

PurposeAddress Range

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• The class-based addressing is also known as the classful model.

Different network classes lend themselves to different network configurations.Different network-to-hosts ratio.

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Some Conventions

• Within a particular network (Class A, B or C), the first and last addresses serve special functions.

The first address represents the network number (for example, 118.0.0.0).The last address represents the directed broadcast address of the network (for example, 118.255.255.255).

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IP Subnetting

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IP Subnet

• Basic concept:A subset of a class A, B or C network.

• IP addresses that do not use subnets consists of

a network portion, and a host portion.

Represents a static two-level hierarchical addressing model.

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IP Subnet (contd.)

• IP subnets introduces a third level of hierarchy.

a network portiona subnet portiona host portion

Allow more efficient (and structured) utilization of the addresses.Uses network masks.

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Natural Masks

• Network mask 255.0.0.0 is applied to a class A network 10.0.0.0.

In binary, the mask is a series of contiguous 1’s followed by a series of contiguous 0’s.11111111 00000000 00000000 00000000

Network portion

Host portion

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Natural Masks (contd.)

• Provide a mechanism to split the IP address 10.0.0.20 into

a network portion of 10, and a host portion of 20.

Decimal BinaryIP address:10.0.0.20 00001010 00000000 00000000 00010100

Mask: 255.0.0.0 11111111 00000000 00000000 00000000

Network Host

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Natural Masks (contd.)

• Class A, B and C addresses Have fixed division of network and host portions.Can be expressed as masks.

Called natural masks.

• Natural MasksClass A :: 255.0.0.0Class B :: 255.255.0.0Class C :: 255.255.255.0

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Creating Subnets using Masks

• Masks are very flexible.Using masks, networks can be divided into smaller subnets.

• How?By extending the network portion of the address into the host portion.

• Advantage gained:We can create a large number of subnets from one network.Can have less number of hosts per network.

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Example: Subnets

• Network mask 255.255.0.0 is applied to a class A network 10.0.0.0.

This divides the IP address 10.5.0.20 into

a network portion of 10, a subnet portion of 5, and a host portion of 20.

The 255.255.0.0 mask borrows a portion of the host space, and applies it to network space.

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Subnets (contd.)

• What happens?Initially it was a single large Class A network (224 – 2 hosts).We have now split the network into 256 subnets.

From 10.0.0.0 to 10.255.0.0.The hosts pet subnet decreases to 65,534.

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Subnets (contd.)

Decimal BinaryIP address: 10.5.0.20 00001010 00000101 00000000 00010100

Mask: 255.255.0.0 11111111 11111111 00000000 00000000

Network Subnet Host

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Default Mask and Subnet mask

AND

AND

Default Mask255.255.0.0

Subnet Mask255.255.192.0

IP Address144.16.72.57

IP Address144.16.72.57

Network Address144.16.0.0

Network Address144.16.64.0

192: 1100 000072: 0100 1000

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Subnets vrs Multiple Address Classes

• SubnetsManagement of subnets is done by local network administrator.Single entry in external router tables.

• Multiple Address ClassesMultiple entries in external router tables.Additional overhead on the backbone (external) routers.

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Comparison

R

R R

R

R R

SUBNETSMULTIPLE ADDRESS CLASSES

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Variable Length Subnet Mask (VLSM)

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Variable Length Subnet Masks (VLSM)

• Basic conceptThe same network can be configured with different masks.Can have subnets of different sizes.Allows better utilization of available addresses.

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Example: VLSM

• Suppose we are assigned a Class C network 192.203.17.0.

To be divided into three subnets.Corresponding to three departments.With 110, 45 and 50 hosts respectively.

D1(110)

D2(45)

D3(50)

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The Example (contd.)

• Available subnet optionsThe network mask will be the Class C natural mask 255.255.255.0Subnet masks of the form 255.255.255.X

Can be used to divide the network into more subnets.

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4641111 11002528321111 1000248

16161111 00002403281110 00002246441100 0000192

12821000 0000128

No. of Hosts

No. of Subnets

X (in binary)

X

• Cannot satisfy the requirements.

The Subnet Options

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The VLSM Option

• Basic concept:Use the mask 255.255.255.128 to divide the network address into two subnets with 128 hosts each.

192.203.17.0 (.0 to .127)192.203.17.0 (.128 to .255)

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The VLSM Option (contd.)

Next subnet the second .128 subnet using a mask of 255.255.255.192.

Creates two subnets, 64 hosts each192.213.17.128 (.128 to .191)192.213.17.128 (.192 to .255)

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The VLSM Option (contd.)

192.203.17.0

192.203.17.0 (.0 to .127) 192.203.17.0 (.128 to .255)

192.213.17.128 (.128 to .191)

192.213.17.128 (.192 to .255)

Mask: 255.255.255.128

Mask: 255.255.255.192

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Interface 1 :: 128 hostsNetwork number: 192.203.17.0

Network mask: 255.255.255.128Address: 192.203.17.0 -- .127

Interface 2 :: 64 hostsNetwork number: 192.203.17.128Network mask: 255.255.255.192Address: 192.203.17.128 -- .191

Interface 3 :: 64 hostsNetwork number: 192.203.17.192Network mask: 255.255.255.192Address: 192.203.17.192 -- .255

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Interface E2 :: 128 hostsNetwork number: 192.203.17.0

Network mask: 255.255.255.128Address range: 192.203.17.0 − .127

Interface E3 :: 64 hostsNetwork number: 192.203.17.128Network mask: 255.255.255.192

Address range: 192.203.17.128 − .191

Interface E4 :: 64 hostsNetwork number: 192.203.17.192Network mask: 255.255.255.192

Address range: 192.203.17.192 − .255

ROUTER

E2

E3 E4

128 Hosts

64 Hosts 64 Hosts

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VLSM :: Current Status

• All routing protocols do not support VLSM.Routing Information Protocol version 1 (RIP-1) do not carry network masks in routing updates.RIP-1 cannot implement VLSM.

• The following protocols support VLSM:Open Shortest Path First (OSPF)RIP-2Enhanced IGRP (EIGRP)

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Classless Internet Domain Routing (Supernetting)

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Classless Inter-Domain Routing (CIDR)

• The size of the global routing tables have grown very fast in recent years.

Caused routers to become saturated.Limits to processing power and available memory.Size of the tables have doubled every 10 months or so, between 1991 and 1995.

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• Without any remedial measure, the routing tables would have grown to about 80,000 routes in 1995.

• But early 2000 data shows that the size was around 76,000.

• Why this reduction?Planned IP address allocation.CIDR.

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Growth of Internet Routing Tables

01000020000300004000050000600007000080000

'88 '94 '96 '98 '00Year

Routing TableSize

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CIDR: Introduction

• CIDR is a new concept to manage IP networks.

Classless Inter Domain Routing.No concept of class A, B, C networks.Reduces sizes of routing tables.

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CIDR: Basic Idea

• An IP address is represented by a prefix, which is the IP address of the network.

• It is followed by a slash, followed by a number M.

M: number of leftmost contiguous bits to be used for the network mask.Example: 144.16.192.57 / 18

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CIDR: An Important Rule

• The number of addresses in each block must be a power of 2.

• The beginning address in each block must be divisible by the number of addresses in the block.

A block that contains 16 addresses cannot have beginning address as 144.16.223.36.But the address 144.16.192.64 is possible.

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Example: CIDR

• An organization is allotted a block with beginning address:

144.16.192.24 / 29What is the range of the block?

Start addr: 10010000 00011000 11000000 00011000End addr: 10010000 00011000 11000000 00011111

There are 8 addresses in the block.

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Example

• Suppose Company A needs IP addresses for 1000 machines

• Assign 4 contiguous Class C address blocks

192.60.128.0192.60.129.0192.60.130.0192.60.131.0

(last 8 bits 0)

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• Supernet:Address : 192.60.128.0Netmask: 255.255.252.0 (last 10 bits 0)

• Also written as: 192.60.128.0/22 22 denotes size of network portion. Also called prefix.Routing done by prefix

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Advantages

• Routing table at higher levels will have only one entry for the 4 networks.

• In classful addressing (that did not recognize masks), would have required 4 entries for the 4 networks.

• Possible only due to contiguous allocation.

Higher level routers can just send it to lower level routers (in this case company A’s router) using one entry only. Lower level router will distinguish.

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• Routing table at all higher level routers:192.60.128.0/22 - send to host X (next hop on way to Company A’s router RA)

• Routing table at RA:192.60.128.0/24 – send to router of first net192.60.129.0/24 – send to router of second net192.60.130.0/24 – send to router of third net192.60.131.0/24 – send to router of fourth net

RA

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• Routers always do longest prefix match. If two entries match, longest match is taken.

Example: two entries in table: one for 192.65.0.0/16 and one for 192.65.128.0/24. If address is 192.65.128.4, second entry will be used even though it matches both.

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Recent Trend

• Move on to CIDR addressing.Existing classful networks can also be represented using this notation.

Class A: W.X.Y.Z / 8Class B: W.X.Y.Z / 16Class C: W.X.Y.Z / 24

• Recent routers support CIDR.