network into subnets can result in better performance and improved use of limited network address
space.
Subnet Mask
The subnet mask is a mechanism used to divide a single IP network into several different
subnetworks. For a given network class, a portion of an IP address that would normally be used to
identify a node is used, instead, to identify a subnetwork. A subnet mask is applied to each IP
address to specify the portion used for subnetworks, and the portion used to identify the node. For
example, see
Table A-3 Example: Subnet Mask 255.255.0.0 Applied to Class A Network.
Table A-3 Example: Subnet Mask 255.255.0.0 Applied to Class A Network
Class A Network Address 15 xxx xxx xxx
Subnet Mask 255 255 0 0
IP Address fields with Subnet
Mask applied
Network Subnet Host Host
Example of a node's IP
Address on Subnet 1
15 1 25 7
Example of a node's IP
Address on Subnet 254
15 254 64 2
As illustrated in Table A-3 Example: Subnet Mask 255.255.0.0 Applied to Class A Network, the Class
A IP network address “15” has been assigned to company ABC. To allow additional networks at
company ABC's site, the subnet mask of 255.255.0.0 is used. This subnet mask specifies that the
second byte of the IP address will be used to identify up to 254 subnets. Using this designation, each
device is uniquely identified on its own subnet, but company ABC can incorporate up to 254
subnetworks without violating their assigned address space.
Gateways
Gateways (routers) are used to connect networks together. Gateways are devices that act as
translators between systems that do not use the same communication protocols, data formatting,
structures, languages, or architectures. Gateways repackage the data packets and change the
syntax to match that of the destination system. When networks are divided into subnets, gateways
are required to connect one subnet to another.
Default Gateway
The Default Gateway is the gateway or router that, if not specified, will be used to move packets
between networks. It is specified by an IP address.
If multiple gateways or routers exist, then the default gateway is typically the address of the first, or
nearest, gateway or router. If no gateways or routers exist, then the default gateway will typically
assume the IP address of the network node (such as the workstation or HP Jetdirect print server).
88 Appendix A TCP/IP Overview ENWW
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