User`s guide

156
TCP/IP Network Basics
IP Addressing
Custom Subnet
Masks
You can create custom subnet masks for Class A, Class B, and Class C
IP addresses. For example, for a Class A IP address, by using custom
subnet mask
255.255.224.0 you can create 2,046 unique networks with
4,094 unique host on each network.
A custom subnet mask adds more bits for the network portion of the IP
address and uses fewer bits for the host portion of the IP address. These
additional bits are called
subnet bits.
Local and Remote Networks
For every packet sent from a source host, IP must determine if the
destination host is on the same
local network or is on a remote network.
If the destination host is on the same local network as the source host,
IP directly gets the hardware address of the destination host. If the
destination host is on a remote network, IP sends packets to the
router.
Example: Creating Local and Remote Networks
For example, consider the two networks (subnets) in this figure that are
created using a router. Computer A, instrument B, instrument C, and one
side of the router form one local network, while computer D, instrument E,
instrument F, and the other side of the router form another local network.
If computer A wants to send a packet to instrument B or to instrument C,
the destination is
local (all hosts are on the same side of the router). In this
case, IP can get the hardware address of the destination host (instrument
B or instrument C) and can establish communication directly with the
destination host.
However, if computer A wants to send a packet to instrument E or to
instrument F, the destination is
remote (hosts are separated by the router).
In this case, IP gets the hardware address of the router. The packet is then
sent to the router for transmission to the destination host (instrument E or
instrument F).
NOTE
Consult a standard network or TCP/IP book or use a subnet calculator
(available on the Web) for creating custom subnets.