Configuring Hardware and Communication Connections STEP 7.pdf

Networking Stations
Relationship between the IP address and the default subnet mask
The relationship between the IP address areas and the so-called "Default Subnet
Masks" is defined. The first decimal of the (left to right) determines the (binary)
structure of the default, that is, the number of "1" values as follows:
IP address (dec.) IP address (bin.) Address class Default subnet mask
0 to 126 0xxxxxxx.xxxxxxxx.... A 255.0.0.0
128 to 191 10xxxxxx.xxxxxxxx... B 255.255.0.0
192 to 223 110xxxxx.xxxxxxxx... C 255.255.255.0
Note
The first decimal number of the IP address can also have a value between 224 and
255 (address class D etc). However, we recommend you do not use these values,
as STEP 7 does not check them.
Forming further subnet masks
The subnet mask allows you to extend the structure of a subnet that is assigned to
address class A, B or C and to create "private" subnets by setting the least
significant bits of the subnet mask to "1". The number of "private" networks will
double with each bit that is set to "1" and reduces the number of network nodes to
half. Seen from the outside this still network has the same effect as a single
network.
Example:
In an address class B subnet (e.g.. IP address 129.80.xxx.xxx) you change the
default subnet mask as follows:
Masks Decimal Binary
Default subnet mask 255.255.0.0 11111111.11111111.00000000.00000000
Subnet mask 255.255.128.0 11111111.11111111.10000000.00000000
Result:
All nodes at the addresses 129.80.001.xxx to 129.80.127.xxx are assigned to the
same subnet, whereas all nodes at the addresses 129.80.128.xxx to
129.80.255.xxx are assigned to a different subnet.
Network gateway (Router)
Network gateways (routers) interconnect subnets. An IP datagram sent to another
network must first be passed on to a router. To enable this feature, you must enter
the router address for each one of the network nodes.
The IP address of a subnet node and of the network gateway (router) may only
differ at the "0" bits of the subnet mask.
Configuring Hardware and Communication Connections with STEP 7
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