Subnets; Subnet Mask - WAGO 750 Series Manual

140 • ETHERNET
Network Communication

Subnets

To allow routing within large networks a convention was introduced in the
specification RFC 950. Part of the Internet address, the subscriber ID is
divided up again into a subnetwork number and the station number of the
node. With the aid of the network number it is possible to branch into internal
subnetworks within the partial network, but the entire network is physically
connected together. The size and position of the subnetwork ID are not
defined; however, the size is dependent upon the number of subnets to be
addressed and the number of subscribers per subnet.
1
1 0
Fig. 5-10: Class B address with Field for Subnet ID

Subnet mask

A subnet mask was introduced to encode the subnets in the Internet. This
involves a bit mask, which is used to mask out or select specific bits of the IP
address. The mask defines the subscriber ID bits used for subnet coding,
which denote the ID of the subscriber. The entire IP address range
theoretically lies between 0.0.0.0 and 255.255.255.255. Each 0 and 255 from
the IP address range are reserved for the subnet mask.
The standard masks depending upon the respective network class are as
follows:
•
•
•
Depending on the subnet division the subnet masks may, however, contain
other values beyond 0 and 255, such as 255.255.255.128 or 255.255.255.248.
Your network administrator allocates the subnet mask number to you.
Together with the IP address, this number determines which network your PC
and your node belongs to.
The recipient node, which is located on a subnet initially, calculates the
correct network number from its own IP address and the subnet mask.
Only then does it check the node number and delivers the entire packet frame,
if it corresponds.
8
Net-ID
Class A Subnet mask:
255 .0
Class B Subnet mask:
255 .255
Class C Subnet mask:
255 .255
16
Subnet-ID
.0 .0
.0 .0
.255 .0
24 32
Host-ID
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP