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IP Address Overview 319
completely internal to the enterprise itself, and seen from the outside, the
enterprise only has one net-id. When an external message enters this enterprise
network, the internal router can route according to the sub-net number, and
finally reach the destination host.
The following figure shows the sub-net classification of a Class B IP address, in
which a sub-net mask consists of a string of continuous “1s” and a string of
continuous “0s”. The 1s corresponds to the network ID field and the sub-net
number field, while the 0s correspond to the host ID field.
Figure 118 Sub-net classification of IP address
Classification of one more sub-net number field is at a price. For example, an IP
address of class B originally consists of 65534 host IDs. But after a 6-bit-long
sub-net field is classified, there may be at most 62 sub-nets (excluding sub-nets
whose numbers are purely 1s or purely 0s). Each sub-net has 10bit host ID, i.e.
each sub-net has 1022 host IDs at most. Totally, there are 62*1022=63364 host
IDs which is less than the sum before sub-net classification.
If there is no sub-net division in an enterprise, then its sub-net mask is the default
value and the length of “1” indicates the net-id length. Therefore, for IP addresses
of classes A, B and C, the default values of corresponding sub-net mask are
255.0.0.0, 255.255.0.0 and 255.255.255.0 respectively.
A router connecting multiple sub-nets will have multiple sub-net IP addresses. The
IP addresses mentioned above cannot be directly used in communication, because:
■ An IP address is only an address of a host in the network layer. To send the data
messages transmitted through the network layer to the destination host,
physical address of the host is required. So the IP address must be first resolved
into a physical address.
■ IP address is hard to remember, but a host domain name will be much easier to
remember and is also more popular. So the host domain name must also be
resolved into an IP address.
The following figure illustrates relation between host name, IP address and
physical address.
net-id
host-id
net-id host-id
11111111 11111111 111111 00 00000000
Local distribution
Subnet-id
Subnet ID Host ID
(a)
(c)
Class B address
Add subnet
number field
Subnet mask
(b)