ip subnetting and variable length subnet masks(vlsms)

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IP SUBNETTING Static Subnet

Variable Length Subnet Masks(VLSMs)

IP Addressing

The Hierarchical IP Addressing Scheme

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IP address consists of 32 bits of information.

You can depict an IP address using one of three methods:

Dotted-decimal, as in 172.16.30.56 Binary, as in 10101100.00010000.00011110.00111000 Hexadecimal, as in AC.10.1E.38

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Network Addressing

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Network Address Range: Class A 00000000 = 0 01111111 = 127

Network Address Range: Class B 10000000 = 128 10111111 = 191

Network Address Range: Class C

11000000 = 192 11011111 = 223

Network Address Ranges: Classes D and E

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The addresses between 224 and 255 are reserved for

Class D and E networks. Class D (224–239) is

used for multicast addresses and Class E (240–255) for scientific purposes,

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Summary of the three classes of networks

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Class A Addresses

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network.node.node.node For example, in the IP address 49.22.102.70, the 49 is the

network address, and 22.102.70 is the node address.

the network address of all 0s (0000 0000) is reserved to designate the default route

Additionally, the address 127, which is reserved for diagnostics, can’t be used either,

you can really only use the numbers 1 to 126 to designate Class A network addresses. This

Class B Addresses

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network.network.node.node

For example, in the IP address 172.16.30.56, the network

address is 172.16, and the node address is 30.56.

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Class C Addresses

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network.network.network.node example IP address 192.168.100.102, the network address is

192.168.100, and the node address is 102.

Private IP Addresses

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These addresses can be used on a private network, but they’re not routable through the Internet.

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การแก้ปัญหาข้อจ ากดัของ IPv4 Addressing

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Network Address Translation

IP Subnetting

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© 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—3-16

IP Subnetting

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CIDR -- Classless InterDomain Routing

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Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of spelling out the bits of the subnet mask, it is simply listed as the number of 1s bits that start the mask. In the above example, instead of writing the address and subnet mask as

192.60.128.0, Subnet Mask 255.255.252.0 the network address would be written simply as: 192.60.128.0/22

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CIDR(2)

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The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24)

Reference http://www.sluug.org/resources/cidr.html

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© 2004 Cisco Systems, Inc. All rights reserved. ICND v2.2—3-32

Variable Length Subnet

Masks (VLSMs)

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What Is a Variable-Length Subnet Mask?

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Subnet 172.16.14.0/24 is divided into smaller subnets

Subnet with one mask (/27)

Then further subnet one of the unused /27 subnets into multiple /30 subnets

Calculating VLSMs

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A Working VLSM Example

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What Is Route Summarization?

Routing protocols can summarize addresses of several networks into one address.

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Summarizing Within an Octet

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Summarizing Addresses in a VLSM-Designed Network

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Implementation Considerations

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• Multiple IP addresses must have the same highest-order bits.

• Routing decisions are made based on the entire address.

• Routing protocols must carry the prefix (subnet mask) length.

Route Summarization Operation in Cisco Routers

Supports host-specific routes, blocks of networks, and default routes

Routers use longest match

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192.16.5.33 /32 Host

192.16.5.32 /27 Subnet

192.16.5.0 /24 Network

192.16.0.0 /16 Block of Networks

0.0.0.0 /0 Default

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Summarizing Routes in a Discontiguous Network

RIPv1 and IGRP do not advertise subnets, and therefore cannot support discontiguous subnets.

OSPF, EIGRP, and RIPv2 can advertise subnets, and therefore can support discontiguous subnets.

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