Two network segments are restricted by distance limitations. Submetting or dividing the network into the segments is also required when localized network management is required for example accounting, sales, customer service departments. There is another reason for the subnetting, which is that the computers on the network, which use more bandwidth, needs to be separated from the rest of the computers. There are certain advantages and disadvantages of the subnetting. Before you start dividing your network into the different segments, you should assign the IP address to each computer in your network segment.

Subnetting makes the network management easier and it is also very helpful for the troubleshooting of a network segment. The internet is a collection of networks where users communication with each other. Each communication on the internet carries the source and the destination address of the computer. This address is called IP address. This 32 bit address has two parts: one part represents the network portion and the other part represents the host portion of the IP address. A company can use some of the bits in the machine or host portion of the address to identify a subnet. In this scenario, the IP address contains three parts: the network address, the subnet address and the machine address.

Subnet Mask Basics

The most recognizable part aspect of subnetting is the Subnet mask. A subnet mask contains 4 bytes, 32 bits and is divided into 4 period separated octets. Typically, a very common subnet mask in binary looks like this.

11111111 11111111 00000000 00000000 255 255 0 0

How to Apply a Subnet Mask

A subnet mask does not work like an IP address and it cannot exist separately without an IP address. An IP address and subnet mask work together to form a network. An IP address splits into two main parts when applying the subnet mask. The leftmost bits of a subnet mask must be set to 1. For example

11111111.00000000.00000000.00000000 11111111.11111111.00000000.00000000 11111111.11111111.11111111.00000000

The above example shows the valid representation of a subnet mask into the binary numbers.

00000000.00000000.00000000.00000000 is an invalid subnet mask. 11111111.11111111.11111111.11111111 is also invalid subnet mask.

All valid subnet masks contain two parts: the left side with all mask bits set to '1' (the extended network portion) and the right side with all bits set to '0' (the host portion), such as the first example above.

Subnetting an IP network can be performed for a variety of reasons such as using the different physical media in an organization, such as FDDI, WAN and Ethernet, preservation of the addresses and for the purpose of security, management and ease of troubleshooting. The most

common reason of the subnetting is to control the network traffic. In an Ethernet network, all computers in a segment see all the packets that are transmitted by all the other computers on the same segment.

In this situation, the network performance can be badly affected due to the heavy traffic loads, collisions and the retransmission of the packets. A router is used to connect the IP networks and it also helps to minimize the load of the traffic.

Subnet Masking

By applying the subnet mask to the IP address you can identify the network and host portion of the IP address. The decimal number 1 represents the network portion in the subnet mask and the node is represented the 0s. Performing a logical AND operation between the IP address and the subnet mask resulting in the network address. For example, using our test IP address and the default Class B subnet mask, we get: 10001100.10110011.11110000.11001000 140.179.240.200 IP address of the class B 11111111.11111111.00000000.00000000 255.255.000.000 Default subnet mask of class B -------------------------------------------------------- 10001100.10110011.00000000.00000000 140.179.000.000 Network Address value

The following example shows the default subnet masks. • Class A Subnet Mask- 255.0.0.0 - 11111111.00000000.00000000.00000000 • Class B Subnet Mask- 255.255.0.0 - 11111111.11111111.00000000.00000000 • Class C Subnet Mask- 255.255.255.0 - 11111111.11111111.11111111.00000000

Subnetting Review

Subnetting allows network and system administrators some flexibility in defining relationship among the hosts of a network. Hosts on the logically and physically different subnets can talk to each other through specialized devices called gateway or router. The ability to filter the traffic between

subnets can make the more bandwidth availability. Subnetting referred to as subdivision of a class based networks into subnetworks.

A router can exchange subnet routes with the other routers in the network. A subnetted network can’t be split into the isolated portion. All the subnets must be contiguous because the routing information cannot be passed to a non-network member. Router can exchange subnet routes with other routers within the network. Since the subnet masks are identical across the network, the routers will interpret these routes in the same manner. However, routers not attached to the subnetted network can't interpret these subnet routes, since they lack the subnet mask.

Therefore, subnet routes are not relayed to routers on other networks. This leads to our second

restriction. Subnetting allows you to create multiple logically different networks within the same class A, B or C. If you break a major network into smaller networks, it allows you to create a network of interconnecting subnetworks. Any device or gateway that is responsible for connecting the different subnetworks must have the distinct IP address one for each subnetwork.

To subnet a network use and extend the natural subnet mask using some of the bits from the host ID portion to create a subnetwork ID. In this example, given a Class C network of the IP address 4.15.5.0 which has a natural subnet mask of 255.255.255.0, you can create subnets in this manner:

11001100.00001111.00000101.00000000 204.15.5.0 11111111.11111111.11111111.11100000 255.255.255.224

---------------------------------|subnet|----

By extending the natural subnet mask to be 255.255.255.224, you have used three bits from the host portion of the mask and used them to make subnets. By using these 3 bits, it is possible to create 8 subnets. The remaining five ID bits of the host portion, each subnet can make 32 host addresses and the 30 addresses out of 32 are assigned to the devices or computers. The host IDs

of all zeros and all ones are not allowed.

204.15.5.0 255.255.255.224 host address range 1 to 30 204.15.5.32 255.255.255.224 host address range 33 to 62 204.15.5.64 255.255.255.224 host address range 65 to 94 204.15.5.96 255.255.255.224 host address range 97 to 126 204.15.5.128 255.255.255.224 host address range 129 to 158 204.15.5.160 255.255.255.224 host address range 161 to 190 204.15.5.192 255.255.255.224 host address range 193 to 222 204.15.5.224 255.255.255.224 host address range 225 to 254

Introduction

This document gives you basic information needed to configure your router for routing IP, such as how addresses are broken down and how subnetting works. You learn how to assign each interface on the router an IP address with a unique subnet. There are many examples to help tie everything together.

Prerequisites

Requirements

There are no specific prerequisites for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Additional Information

If definitions are helpful to you, use these vocabulary terms to get you started:

• Address—The unique number ID assigned to one host or interface in a network.

• Subnet—A portion of a network sharing a particular subnet address.

• Subnet mask—A 32-bit combination used to describe which portion of an address refers to the subnet and which part refers to the host.

• Interface—A network connection.

If you have already received your legitimate address(es) from the Internet Network Information Center (InterNIC), you are ready to begin. If you do not plan to connect to the Internet, Cisco strongly suggests that you use reserved addresses from RFC 1918 .

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Understanding ip Addresses

An IP address is an address used to uniquely identify a device on an IP network. The address is made up of 32 binary bits which can be divisible into a network portion and host portion with the help of a subnet mask. The 32 binary bits are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal and separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, 172.16.81.100). The value in each octet ranges from 0 to 255 decimal, or 00000000 - 11111111 binary.

Here is how binary octets convert to decimal: The right most bit, or least significant bit, of an octet holds a value of 2⁰. The bit just to the left of that holds a value of 2¹. This continues until the left-most bit, or most significant bit, which holds a value of 2⁷. So if all binary bits are a one, the decimal equivalent would be 255 as shown here:

1 1 1 1 1 1 1 1

128 64 32 16 8 4 2 1 (128+64+32+16+8+4+2+1=255)

Here is a sample octet conversion when not all of the bits are set to 1.

0 1 0 0 0 0 0 1

0 64 0 0 0 0 0 1 (0+64+0+0+0+0+0+1=65)

And this is sample shows an IP address represented in both binary and decimal.

10. 1. 23. 19 (decimal)

00001010.00000001.00010111.00010011 (binary)

These octets are broken down to provide an addressing scheme that can accommodate large and small networks. There are five different classes of networks, A to E. This document focuses on addressing classes A to C, since classes D and E are reserved and discussion of them is beyond the scope of this document.

Note: Also note that the terms "Class A, Class B" and so on are used in this document to help facilitate the understanding of IP addressing and subnetting. These terms are rarely used in the industry anymore because of the introduction of classless interdomain routing (CIDR).

Given an IP address, its class can be determined from the three high-order bits. Figure 1 shows the significance in the three high order bits and the range of addresses that fall into each class. For informational purposes, Class D and Class E addresses are also shown.

Figure 1

In a Class A address, the first octet is the network portion, so the Class A example in Figure 1 has a major network address of 1.0.0.0 - 127.255.255.255. Octets 2, 3, and 4 (the next 24 bits) are for the network manager to divide into subnets and hosts as he/she sees fit. Class A addresses are used for networks that have more than 65,536 hosts (actually, up to 16777214 hosts!).

In a Class B address, the first two octets are the network portion, so the Class B example in Figure 1 has a major network address of 128.0.0.0 - 191.255.255.255. Octets 3 and 4 (16 bits) are for local subnets and hosts. Class B addresses are used for networks that have between 256 and 65534 hosts.

In a Class C address, the first three octets are the network portion. The Class C example in Figure 1 has a major network address of 192.0.0.0 - 233.255.255.255. Octet 4 (8 bits) is for local subnets and hosts - perfect for networks with less than 254 hosts.