The reasons behind sub-networking date back to the early specification of
IP - where just a few sites were running on Class A network numbers,
which allow for millions of connected hosts.
It is obviously a huge traffic and administration problem if all IP
computers at a large site need to be connected to the same network:
trying to manage such a huge beast would be a nightmare and the network
would (almost certainly) collapse under the load of its own traffic
Enter sub-networking: the A class IP network address can be split up to
allow its distribution across several (if not many) separate networks.
The management of each separate network can easily be delegated as well.
This allows small, manageable networks to be established - quite
possibly using different networking technologies. Remember, you cannot mix
Ethernet, Token Ring, FDDI, ATM etc on the same physical network - they
can be interconnected, however!
Other reasons for sub-networking are:-
- Physical site layout can create restrictions (cable run lengths)
in terms of the how the physical infrastructure can be connected,
requiring multiple networks. Sub-networking allows this to be done in an
IP environment using a single IP network number.
This is in fact now very commonly done by ISPs who wish to give their
permanently connected clients with local networks static IP numbers.
- Network traffic is sufficiently high to be causing significant
slow downs. By splitting the network up using subnetworks, traffic that
is local to a network segment can be kept local - reducing overall
traffic and speeding up network connectivity without requiring more
actual network bandwidth;
- Security requirements may well dictate that different classes of
users do not share the same network - as traffic on a network can always
be intercepted by a knowledgeable user. Sub-networking provides a way to
keep the marketing department from snooping on the R & D network traffic
(or students from snooping on the administration network)!
- You have equipment which uses incompatible networking technologies
and need to interconnect them (as mentioned above).