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142 APPENDIX A: NETWORKING PRIMER
A private organization’s internal router uses this new breakdown of (longer)
network number and (shorter) host number to route data to subdivided areas, or
subnets, within the broader “network” (as identified by the original network
number). The temporary, longer network number identifies the subnet within the
broader network, and the temporary, shorter host number identifies the exact
location of a host within that subnet.
The subnet mask makes this temporary, alternate reading of the IP address
possible within the bounds of the private organization, while at the same time
public routers continue to read the same IP address (without the subnet mask) to
route data to the broader, network level. In other words, a subnet mask allows a
single IP address to serve double duty.
Why use such a complex mechanism? The complicated answer has to do with the
need to solve IP address space shortage problems caused by limitations of the
original IP Class structures, without compromising the efficiency of the entire
system or disrupting it completely.
Because Internet routers use permanent routing tables to keep track of every
(public) IP address down to the network level, the efficiency of Internet routing
cannot be maintained if these tables have to change and expand constantly. At
the same time, if an organization grows beyond a certain number of hosts
(historically, this has occurred with Class C networks, which have a 254 -host
limit), it must either obtain a second public IP address (which in turn expands the
Internet routing tables and causes them to have to be updated), or it must be able
to extend the use of the IP address it already possesses to accommodate a larger
number of hosts.
Subnetting became a way to allow for the latter, and subnet masks were devised
as a way of accomplishing this within the existing framework, using
already-assigned IP addresses. Since Internet routers only route to the level of
networks, all that goes on with a subnet mask and the new network number
derived through its use remains invisible to an Internet router and does not
impede its speed and efficiency. In addition, if an organization has to restructure
its IP address subnetting scheme internally, this can be done without affecting the
permanent, Internet routing tables.
When a pattern of eight bit of all-one’s is converted to its decimal equivalent, the
resulting number is 255. A pattern of eight bits of all-zero’s yields 0. This is why
you frequently see subnet masks such as 255.255.255.0.
Dynamic and Static IP
Addresses
IP addresses for public networks must be unique and provided by the Network
Information Center (NIC). Because of the increasing popularity of the Internet, the
NIC has begun to run out of permanent IP addresses. It has therefore become
common practice to use dynamic IP addresses, which are assigned temporarily and
then reused, instead of static IP addresses which are permanently assigned. For
example, when accessing the Internet, your ISP has a pool of IP addresses it uses to
provide temporary connections to multiple users. Once you disconnect from the
Internet, the IP address you were using is placed back into the pool of addresses
for use by another user.