INTRODUCTION TO IPv4

Internet Protocol version 4 is the fourth version in the development of Internet Protocol (IP)

and the first version of the protocol to be widely used. It is one of the core protocols of the

standards-based internetworking methods of the Internet, and routes most traffic in the

Internet. IPv4 is a connectionless protocol for use on packet-switched networks.

IPv4 uses 32-bit (4-byte) addresses, which limits the address space to 4,294,967,296 (232)

addresses. As addresses were assigned to users, the number of unassigned addresses

decreased. IPv4 address exhaustion occurred on February 3, 2011, although it had been

significantly delayed by address changes such as classful network design, Classless Inter-

Domain Routing, and network (NAT).

IPv4 reserves special address blocks for private networks (~18 million addresses)

and multicast addresses (~270 million addresses).

ADDRESS REPRESENTATION IN IPv4

IPv4 addresses may be written in any notation expressing a 32-bit integer value, but for

human convenience, they are most often written in the dot-decimal notation, which consists

of 4 octets of the address expressed individually in decimal and separated by decimal point or

periods (.).

For example, the IP address 00000101.00001010.00000001.00001101 is represented in

dotted decimal notation as 5.10.1.13

The IPv4 address ranges from 0.0.0.0 (00000000.00000000.00000000.00000000) to

255.255.255.255 (11111111.11111111.11111111.11111111).

This limitation of IPv4 stimulated the development of IPv6 in the 1990s, which has been in

commercial deployment since 2006. The address shortage problem is aggravated by the fact

that portions of the IP address space have not been efficiently allocated. Also, the traditional

model of classful addressing does not allow the address space to be used to its maximum

potential. As the new markets open and a significant portion of the world population become

candidates for IP addresses, the finite number of IP addresses will eventually be exhausted.

In order to provide the flexibility required to support different size networks, the designers

decide to divide the IP address space into different address classes – Class A, Class B and

Class C. this is often referred to as ‘classful’ addressing because the address space is split into

3 predefined classes, groupings or categories. Each class fixes the boundary between the

network-prefix and host-number at a different point within the 32-bit address.

One of the fundamental features of classful IP addressing is that each address contains a self-

encoding key that identifies the dividing point between the network-prefix and the host-

number.

Class A Networks (/8 prefixes)

Each Class A network address has an 8-bit network-prefix with the MSB set to 0 and a 7-bit

network number, followed by a 24-bit host-number. Class A networks are referred to as ‘/8’

since they have an 8-bit network-prefix. A maximum of 126 (27 – 2) /8 networks can be

defined. 2 is subtracted from 27 because the address 0.0.0.0 has been reserved for use as the d

default route and the address 127.0.0.0 has been reserved for ‘loopback’ function. Maximum

number of hosts per network in /8 is 224 – 2. IP address range is from 0.0.0.0 to

127.255.255.255, where 0.0.0.0 is the IP address of the network itself and 127.255.255.255 is

used for broadcasting purpose.

Class B Networks (/16 prefixes)

Each Class B network address has a 16-bit network-prefix with the first 2 highest order bits

set to 10 and a 14-bit network number, followed by a 16-bit host-number. Class B networks

are referred to as ‘/16’ since they have a 16-bit network-prefix. A maximum of 16,384 (214)

/16 networks can be defined. Maximum number of hosts per network in /16 is 216 – 2. IP

address range is from 128.0.0.0 to 191.255.255.255, where 128.0.0.0 is the IP address of the

network itself and 191.255.255.255 is used for broadcasting purpose.

Class C Networks (/24 prefixes)

Each Class C network address has a 24-bit network-prefix with the first 3 highest order bits

set to 110 and a 21-bit network number, followed by an 8-bit host-number. Class C networks

are referred to as ‘/24’ since they have a 24-bit network-prefix. A maximum of 221 /24

networks can be defined. Maximum number of hosts per network in /24 is 254 (28 – 2). IP

address range is from 192.0.0.0 to 223.255.255.255, where 192.0.0.0 is the IP address of the

network itself and 223.255.255.255 is used for broadcasting purpose.

There is also a Class D network which is used for multicasting and a Class E network for

research purposes and future use.

SUBNETTING

Subnetting and Supernetting are the techniques used to make up for the shortage of IP

addresses.

Subnetting is the procedure of dividing a single Class A, B or C network into smaller pieces.

Subnetting was introduced to overcome some of the problems that parts of the Internet were

beginning to experience with the classful 2-level addressing hierarchy.

Subnetting attacked the expanding routing table problem by ensuring that the subnet structure

of a network is never visible outside of the organization’s private network. The route from the

Internet to any subnet of a given IP address is the same, no matter which subnet the

destination host is on. This is because all subnets of a given network use the same network-

prefix but different subnet numbers. The routers within the private organization are collected

into a single routing table entry. This allows the local administrator to introduce arbitrary

complexity into the private network without affecting the size of Internet’s routing tables.

Subnetting overcame the registered number issue by assigning each organization one (or at

most a few) network number(s) from the IPv4 address space. The organization is then free to

assign a distinct subnet number for each of its internal networks.

There are two types of subnetting used:

1. Variable Length Subnet Mask (VLSM)

2. Classless Inter Domain Routing (CIDR)

A subnet mask is denoted as <IP address>/n, where ‘n’ denotes the number of bits used to

identify the type of network.

Rules to write Subnet Mask:

‘n’ bits from the MSB end is set to 1 and the remaining (32 – n) bits are set to 0, and then it is

written in dotted-decimal format.

For Class A, default subnet mask is /8 i.e. 11111111.00000000.00000000.00000000, which

when written in dotted-decimal gives 255.0.0.0.

For Class B, default subnet mask is /16 i.e. 11111111.11111111.00000000.00000000, which

when written in dotted-decimal gives 255.255.0.0.

For Class C, default subnet mask is /24 i.e. 11111111.11111111.11111111.00000000, which

when written in dotted-decimal gives 255.255.255.0.

The standards describing modern routing protocols often refer to the extended network-prefix

length rather than the subnet mask. The prefix length is equal to the number contiguous 1-bits

in the traditional subnet mask. This means that specifying a network address 130.5.5.25 with

a subnet mask of 255.255.255.0 can also be expressed as 130.5.5.25/24. The /<prefix-length>

notation is more compact and easier to understand than writing out the mask in its traditional

dotted-decimal format.

1. CLASSLESS INTER DOMAIN ROUTING (CIDR)

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and

routing Internet Protocol packets. The Internet Engineering Task Force (IETF)

introduced CIDR in 1993 to replace the previous addressing architecture of classful

network design in the Internet. Its goal was to slow the growth of routing tables on

routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.

Classful network design for IPv4 sized the network address as one or more 8-bit

groups, resulting in the blocks of Class A, B or C addresses. Classless Inter-Domain

Routing allocates address space to Internet Service Providers (ISPs) and end users on

any address bit boundary, instead of on 8-bit segments.

CIDR eliminates the traditional concept of Class A, B and C network addresses, and

supports route aggregation where a single routing table entry can represent the address

space of perhaps thousands of traditional classful routers. Route aggregation helps

control the amount of routing information in the Internet’s backbone routers, reduces

route flapping (rapid changes in route availability), and eases the local administrative

burden of updating external routing information. Without the rapid deployment of

CIDR in 1994 and 1995, the Internet routing tables would have in excess of 70,000

routes (instead of the current 30,000+) and the Internet would probably not be

functioning today.

For example, /19 is written in binary as 11111111.11111111.11100000.00000000. In

dotted-decimal, it is 255.255.224.0.

Routers that support CIDR do not make assumptions based on the first 3-bits of the

address, rather they rely on the prefix-length information provided with the route.

Q. You have assigned a network address of 192.168.0.0/24. You need to create subnet

network IDs for 4 different subnets. You want to use a subnet mask that provides the

greatest number of hosts in each sub-network. Determine the network ID, Broadcast

ID, Subnet Mask, IP range for each sub-network.

A. Network-prefix is /24. To create 4 subnets in the given network, 2-bits are needed

to identify and distinguish 4 sub-networks from one another. Hence, 2-bits are

borrowed from the last octet of the IP address to make the Subnet Mask as /26.

Total number of IP addresses to be allocated is 256 (192.168.0.0 to 192.168.0.255).

Hence, each of the 4 sub-networks will contain 64 hosts, thereby having 64 IP

addresses.

Sub-

Network #

#1 #2 #3 #4

Network ID 192.168.0.0 192.168.0.64 192.168.0.128 192.168.0.192

Broadcast

ID

192.168.0.63 192.168.0.127 192.168.0.191 192.168.0.255

Subnet

Mask (/26)

255.255.255.192 255.255.255.192 255.255.255.192 255.255.255.192

IP Range 192.168.0.1 to

192.168.0.62

192.168.0.65 to

192.168.0.126

192.168.0.129 to

192.168.0.190

192.168.0.193 to

192.168.0.254

2. VARIABLE LENGTH SUBNET MASKS (VLSM)

1n 1987, RFC 1009 specified how a subnet network could use more than one subnet

masks. When an IP network is assigned more than one subnet masks, it is considered

as a network with ‘variable length subnet masks’.

Multiple subnet masks permit more efficient use of an organization’s assigned IP

address space. Multiple subnet masks also permits route aggregation which can

significantly reduce the amount of routing information at the ‘backbone’ level within

an organization’s routing domain. VLSM supports more efficient use of an

organization’s assigned IP address space.

One of the major problems with the earlier limitation of using only a single subnet

mask across a given network-prefix was that once the mask was selected, it locked the

organization into a fixed number of fixed sized subnets.

Conceptually, a network is first divided into subnets, some of the subnets are further

divided into sub-subnets, and some of the sub-subnets are further divided into sub 2-

nets. This allows the detailed structure of routing information for one subnet group to

be hidden from routers in another subnet group.

Q. For a Class C network 202.195.32.0 assigned to ISP. Determine the network ID,

broadcast ID, subnet mask & IP range for each sub-network from the given topology.

A. Default subnet mask for class C is /24.

Bit Value 128 64 32 16 8 4 2 1

Bits

Borrowed

1 2 3 4 5 6 7 8

Subnet Mask /25 /26 /27 /28 /29 /30

Beginning with network E (100 hosts), from the above table, we see that we are

borrowing 1 bit with the value of 128 (which is closest to 100). Therefore, the subnet

mask will be /25. We need 7 bits to identify each host in network E. Therefore,

maximum number of hosts in network E is 128 (27).

In network A (50 hosts), from the above table, we see that we are borrowing 2 bits

with the value of 64 (which is closest to 50). Therefore, the subnet mask will be /26.

We need 6 bits to identify each host in network A. Therefore, maximum number of

hosts in network A is 64 (26).

In network B (13 hosts) and C (14 hosts), from the above table, we see that we are

borrowing 4 bits with the value of 16 (which is closest to 13 and 14). Therefore, the

subnet mask for B and C will be /28. We need 4 bits to identify each host in networks

B and C. Therefore, maximum number of hosts in B and C is 16 (24) each.

Since network D is the only one left, the rest all 16 host addresses are given to it.

Sub-Network Network ID Broadcast ID Subnet Mask IP Range

A 202.195.32.128 202.195.32.191 /26

255.255.255.192

202.195.32.129 to

202.195.32.190

B 202.195.32.208 202.195.32.223 /28

255.255.255.240

202.195.32.209 to

202.195.32.222

C 202.195.32.192 202.195.32.207 /28 202.195.32.193 to

255.255.255.240 202.195.32.206

D 202.195.32.224 202.195.32.239 /28

255.255.255.240

202.195.32.225 to

202.195.32.238

E 202.195.32.0 202.195.32.127 /25

255.255.255.128

202.195.32.1 to

202.195.32.127

CONCLUSION

Therefore, it is seen that by using subnetting and its different techniques, we are able to use

IPv4 addresses. But once they are totally exhausted, then we will be deployed to 128-bit

IPv6, which is far more reliable and has a very large IP address space as compared to the

address space of IPv4.

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