IP
Address Classes or Classful addressing
• IP
addressing supports five different address classes: A, B, C, D and E. Only
classes A, Band C are available for commercial use.
• We can
find the class of an address when given the address in binary notation or
dotted decimal notation.
• If the
address is given in binary notation, the first few bits can tell us the class
of the address.
• If
the address is given in dotted decimal notation, the first byte defines the
class.
Class A
addresses
1. Class
A addresses are designed for large organizations with a large number of hosts
or routers.
2. In
this the first octet of the address identifies the network and the next three
octets are used to identify the host.
3. The
first bit of first octet is always 0 and the remaining 7 bits are used to
identify the network address.
4. The
next three octets i.e. 24 bits are used to identify the host.
5. The
class support addresses from 0.0.0.0 to 0.255.255.255
6. The
first block of network address starts with 1.0.0.0 and the last block of
network address starts with 127.0.0.0.
7. As
there are 7 bits in network address, 27 = 128 blocks of network
address are possible. Out of these two network blocks are reserved. Hence total
126 address blocks are used.
8. Each
network blocks can have 224— 2 hosts i.e. 16,777,214
host address. Two addresses are less as one address is reserved for the
broadcast address and one address is reserved for the network.
9. A block
in class A is too large for almost any organization. This means most of the
addresses in class A are wasted and are not used.
Class B
address
1. The
class B addresses are designed for medium sized organizations with tens of
thousands of attached hosts or routers.
2. In
this, the first two octets of the address identify the network and the next two
octets identify the host within the network.
3. The
first two bits (high order bits) of first octet are always 1,0. Thus the
remaining 14 bits identify the network
4. The
third and fourth octet i.e. 6 bits are used to identify the
host.
5. The
first network block of this class covers the addresses from 128.0.0.0 to
128.0.255.255 (net id 128.0). The last network block of this class covers
addresses from 191.255.255.255 (net id 191.255)
6. The
maximum number of network blocks in class B is 214 = 16384.
7 Each
network block in class B can have 216— 2 = 65,534 hosts.
8. A
block in class B is also very large and most of the address in class B is also
wasted.
Class C
address
1. The
class C addresses is designed for small organizations with a small number of
attached hosts or routers.
2. In
class C, the first three octets of address are used for network and the last
octet is used to identify the host.
3. The
first three bits of first octet are always set to 1, 1, 0.
4. The
remaining 24 – 3 = 21 bits are used for network identification and only 8 bits
are used for host.
5. In
class C, 221 = 2,097,152 network blocks are possible.
6. Thus,
each block in class C address can have 28 – 2 = 254 hosts.
7. The
first block of network covers addresses from 192.0.0.0 to 192.0.0.255.
The last
block of network covers the addresses form 223.255.255.0 to 223.255.255.255
8. The
class C addresses are too less for many organizations as it supports only 254
hosts in a network.
Class D
address
1. Class
D addresses are used for multicast groups (multi-casting)
2. The
concept of division of octets into network id and host id does not apply to
class D.
3. The
first four bits of first octet in class D are always set to 1,1,1,0.
4. The
address range is 224.0.0.0 to 239.255.255.255
Class E
address
1. The
Class E address are reserved for future use and are experimental.
2. The
concept of network id and host-id does not apply on class E also.
3. The
first four bits of first octet are always set to 1,1,1,1.
4. The
address range for class E is 240.0.0.0 to 255.255.255.255.
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