- Freecom Router User Manual

Freecom FSG-3 Storage Gateway

Subnet Addressing

You can see that even with a Class C address, there are a large number of hosts

per network. It would be a very inefficient use of the amount of IP addresses to

supply every LAN with so many IP addresses. A smaller office LANs does not have

that many devices. A more efficient technique is known as subnet addressing.

Subnet addressing allows us to split one IP network address into smaller multiple

physical networks known as subnetworks. Some of the node numbers are used

as a subnet number instead. A Class B address gives us 16 bits of node numbers

translating to 64,000 nodes. Most organizations do not use 64,000 nodes, so

there are free bits that can be reassigned. Subnet addressing makes use of those

bits that are free.

A Class B address can be effectively translated into multiple Class C addresses. For

example, the IP address of 172.16.97.235 would be interpreted as IP network

address 172.16, subnet number 97, and node number 235. In addition to

extending the number of addresses available, subnet addressing provides other

benefits. Subnet addressing allows a network manager to construct an address

scheme for the network by using different subnets for other geographical

locations in the network or for other departments in the organization.

Although the preceding example uses the entire third octet for a subnet address,

note that you are not restricted to octet boundaries in subnetting. To create

more network numbers, you only need to shift some bits from the host address

to the network address. For instance, to partition a Class C network number

(192.68.135.0) into two, you shift one bit from the host address to the network

address. The new netmask (or subnet mask) is 255.255.255.128. The first subnet

has network number 192.68.135.0 with hosts 192.68.135.1 to 129.68.135.126,

and the second subnet has network number 192.68.135.128 with hosts

192.68.135.129 to 192.68.135.254.

Attention: The number 192.68.135.127 is not assigned because it

is the broadcast address of the first subnet. The number

192.68.135.128 is not assigned because it is the network address

of the second subnet.

Freecom strongly recommends that you configure all hosts on a LAN segment to

use the same netmask for the following reasons:

• So that hosts recognize local IP broadcast packets

When a device broadcasts to its segment neighbors, it uses a destination

address of the local network address with all ones for the host address. In

order for this scheme to work, all devices on the segment must agree on

which bits comprise the host address.

• So that a local router or bridge recognizes which addresses are local and

which are remote Private IP Addresses