sub netting subnet operations

8/8/2019 Sub Netting Subnet Operations

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Classless Interdomain Routing (CIDR)

You can think of the Internet as one big network. As such, each device on the network needs its own unique IP

address. In the early days of the Internet, every device would receive a registered IP address. As the Internet

grew, however, it became apparent that the number of hosts would quickly exceed the number of possible IP

addresses.

One solution to the problem is Classless Interdomain Routing (CIDR). Classfulladdresses are IP addresses that

use the default subnet mask. They are classfull because the default subnet mask is used to identify the

network and host portions of the address. Classless addresses are those that use a custom mask value to

separate network and host portions of the IP address. CIDR allows for variable length subnet masking (VLSM)

and enables the following features:

y Subnetting, dividing a network address into multiple smaller subnets. For example, this allows a singleClass B or Class C addresses to be divided and used by multiple organizations.

y Supernetting, combining multiple network addresses into a single larger subnet. For example, thisallows multiple Class C addresses to be combined into a single network.

y Route aggregation (also called route summarization), where multiple routes are combined in a routingtable as a single route.

CIDR routers use the following information to identify networks.

y The beginning network address in the rangey The number of bits used in the subnet mask

For example, the routing table represents the address as 199.70.0.0/21, where 21 is the number of bits in the

custom subnet mask.

In addition to CIDR, the following other solutions were put into place to make efficient use of available IP

addresses:

y IP version 6. IPv6 uses 128-bit addresses instead of the 32-bit addresses used with IPv4.y Private addressing with address translation. With private addressing, hosts are assigned an

unregistered address in a predefined range. All hosts on the private network use a single registered IP

address to connect to the Internet. A special router (called a Network Address Translation or NAT

router) translates the multiple private addresses into the single registered IP address.


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Binary Calculations

y To perform subnetting operations, you will need to be proficient at converting decimal and binarynumbers. When working with IP addresses, work with each octet separately. The following table shows

the decimal value for various binary values with a single 1 bit.

Binary Value 10000000 01000000 00100000 00010000 00001000 00000100 00000010 00000001

Decimal Value 128 64 32 16 8 4 2 1

y To find the decimal value of a number with multiple 1 bits, simply add the decimal value of the bitstogether. For example, the decimal value of the binary number 10010101 is:

y 10000000 = 12800010000 = 16

00000100 = 4

00000001 = 1

Total = 128 + 16 + 4 + 1 = 149

y To calculate the number of valid subnets or the number of hosts per subnet, you will need to knowhow to find the exponential values of 2. Use the following chart to identify the exponent values andthe final possible number (after subtracting 2 from each exponent).

# of bits 1 2 3 4 5 6 7 8 9 10 11 12

Exponent 21

22

23

24

25

26

27

28

29

210

211

212

Exponent value 2 4 8 16 32 64 128 256 512 1024 2048 4096

Total number (-2) 0 2 6 14 30 62 128 254 510 1022 2046 4094

y Tip: Memorize the shaded values. To find smaller or larger values, divide or multiply the exponentvalue by 2.


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Subnetting Operations - Use the following chart to identify the solutions to common subnetting tasks.

Scenario Solution

Given a network address and subnet

mask, how many subnets can you have?

Begin by converting the subnet mask to a binary number. Then

decide which formula to use (n is the number of additional bits

borrowed from the default mask):

Use 2n

if:

y The network uses a classless routing protocol, such as RIPversion 2, EIGRP, or OSPF

y The ip subnet zero command is configuredy Variable-length Subnet Mask (VLSM) is used

Use 2n-2 if:

y The network uses a classful routing protocol, such as RIPversion 1 or IGRP

y The no ip subnet zero command is configuredNote: If no network details are provided, use 2

n.

Given a network address and subnet

mask, how many hosts per subnet can

you have?

2n-2

Begin by converting the subnet mask to a binary number. Then use

the formula to find the number of hosts.

To find the number of valid hosts, n = the number of unmasked bits

by the custom mask.

Given a network address and customerrequirements, what subnet mask should

you use?

2n

,2n

-2Write out the default subnet mask in binary. Then borrow bits and

use the formula to find the number that gives you enough subnets

and hosts.

Given a network address and a subnet

mask, identify the valid subnet

addresses.

Magic number

The magic number is the decimal value of the last 1 bit in the

subnet mask.

The magic number identifies:

y The first valid subnet addressy The increment value to find additional subnet addresses

Given an IP address and subnet mask,

find the:

y Subnet addressy Broadcast addressy Valid host address range

Trust the line

Use the following process to find the information you need:

1. Identify the subnet and host portions of the mask, draw aline

2. To find the subnet address, set all host bits to 03. To find the broadcast address, set all host bits to 14. The valid host range is:


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o First address = Subnet address + 1o Last address = Broadcast address - 1

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