Classification of ipv4

IPv4 - Address Classes

Internet Protocol hierarchy contains several classes of IP Addresses to be used efficiently in various situations as per the requirement of hosts per network. Broadly, the IPv4 Addressing system is divided into five classes of IP Addresses. All the five classes are identified by the first octet of IP Address.

Internet Corporation for Assigned Names and Numbers is responsible for assigning IP addresses.

The first octet referred here is the left most of all. The octets numbered as follows depicting dotted decimal notation of IP Address −

IP Decimal Notation

The number of networks and the number of hosts per class can be derived by this formula −

Number of networks

When calculating hosts' IP addresses, 2 IP addresses are decreased because they cannot be assigned to hosts, i.e. the first IP of a network is network number and the last IP is reserved for Broadcast IP.

Class A Address

The first bit of the first octet is always set to 0 (zero). Thus the first octet ranges from 1 – 127, i.e.

Class A Addresses

Class A addresses only include IP starting from 1.x.x.x to 126.x.x.x only. The IP range 127.x.x.x is reserved for loopback IP addresses.

The default subnet mask for Class A IP address is 255.0.0.0 which implies that Class A addressing can have 126 networks (27-2) and 16777214 hosts (224-2).

Class A IP address format is thus: 0NNNNNNN.HHHHHHHH.HHHHHHHH.HHHHHHHH

Class B Address

An IP address which belongs to class B has the first two bits in the first octet set to 10, i.e.

Class B Addresses

Class B IP Addresses range from 128.0.x.x to 191.255.x.x. The default subnet mask for Class B is 255.255.x.x.

Class B has 16384 (214) Network addresses and 65534 (216-2) Host addresses.

Class B IP address format is: 10NNNNNN.NNNNNNNN.HHHHHHHH.HHHHHHHH

Class C Address

The first octet of Class C IP address has its first 3 bits set to 110, that is −

Class C Addresses

Class C IP addresses range from 192.0.0.x to 223.255.255.x. The default subnet mask for Class C is 255.255.255.x.

Class C gives 2097152 (221) Network addresses and 254 (28-2) Host addresses.

Class C IP address format is: 110NNNNN.NNNNNNNN.NNNNNNNN.HHHHHHHH

Class D Address

Very first four bits of the first octet in Class D IP addresses are set to 1110, giving a range of −

Class D Addresses

Class D has IP address range from 224.0.0.0 to 239.255.255.255. Class D is reserved for Multicasting. In multicasting data is not destined for a particular host, that is why there is no need to extract host address from the IP address, and Class D does not have any subnet mask.

Class E Address

This IP Class is reserved for experimental purposes only for R&D or Study. IP addresses in this class ranges from 240.0.0.0 to 255.255.255.254. Like Class D, this class too is not equipped with any subnet mask.

 




3.5.1 IPv4 Addresses

The allocation of addresses on the Internet is controlled by the Internet Assigned Numbers Authority (IANA), although authority is delegated to several local registries. IPv4 addresses may be interpreted in two ways. Initially, they were divided into distinct ranges of addresses called classes, but this proved to be inflexible and now a more flexible scheme, called classless addressing, dominates IPv4 internetworks. I shall describe both ways of interpreting IPv4 addresses because the limitation of the first interpretation leads naturally to the need for the second interpretation. The formats of the four allocated classes of IPv4 addresses are shown in Figure 21 and their ranges are listed in Table 6. The values in the table follow the standard dotted-decimal convention of splitting each 32-bit address into four bytes, and expressing the values of the bytes as denary numbers separated by full stops.

Figure 22
Figure 22 IPv4 Internet address formats
Table 6: IPv4 Internet address ranges
ClassRange
LowestHighest
A0.0.0.0127.255.255.255
B128.0.0.0191.255.255.255
C192.0.0.0223.255.255.255
D224.0.0.0239.255.255.255

Addresses in the range 240.0.0.0 to 255.255.255.255 are reserved for future use.

IPv4 address classes A to C identify single hosts, and are called unicast addresses. Looking at Figure 22, you can see that the IPv4 addresses consist of three parts: a prefix identifying the class of address, the address of a network, and the address of a host within the network. For instance, a class C address is identified by the prefix 110, the network address consists of 21 bits, and the host address consists of eight bits. This means that, in principle, there can be 221 =  2097152 networks in an internetwork with class C addresses, and each single network can have 28 =  256 hosts connected to it. In practice, some addresses are reserved and cannot be allocated to networks or hosts.

SAQ 11

To which class does the IPv4 address 157.107.34.5 belong?

Class D addresses are allocated to groups of hosts rather than single hosts. The group is identified by a single 28-bit address, called a multicast address, and the datagram is delivered to all the hosts in the group. Some multicast addresses are reserved for specific groups of hosts, such as all routers connected to a LAN. Other multicast





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