ELSA LANCOM Business 6000, Manual, Page: 115 / 174

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Technical basics
115
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ELSA LANCOM Business 6000
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EN
Network mask How then can you differentiate between the part that determines the
network and the part that identifies the host? With the netmask. You know
what masks are: they cover up one part of something and only allow a
different part to be visible. This is exactly how a netmask operates. It is a
number which is identical in structure to the IP address, i.e. 32 zeros or ones.
The netmask usually starts with ones at the beginning and ends with zeros.
The zeros at the end thus cover the part of the IP address which does not
belong to the network address.
Examples:
The same IP address, this time with another netmask:
You can see from this that an IP address alone is not enough. A host can only
be identified unambiguously in combination with a netmask.
And you can also see that there are more bits available to identify the
individual hosts in a connected network if there are fewer bits in a netmask
that contain a one. While only 254 different addresses could be allocated in
the first example with the network mask 255.255.255.0 (the final numbers
'254' and ' 64.516' are reserved), the second example has as many as 254 x
254 = different addresses available! The first and the last digit of an address
space are reserved for the network address and the broadcast address
(addresses for packets to all hosts in an IP network). In the netmask
255.255.255.0 this is the '0' for the network address and the '255' for the
broadcast address.
This address... ...in bytes... ...looks like this in bits:
IP address 192.168.120.253 11000000.10101000.01111000.11111101
Network mask 255.255.255.0 11111111.11111111.11111111.00000000
Network address 192.168.120.0 11000000.10101000.01111000.00000000
This address... ...in bytes... ...looks like this in bits:
IP address 192.168.120.253 11000000.10101000.01111000.11111101
Network mask 255.255.0.0 11111111.11111111.00000000.00000000
Network address 192.168.0.0 11000000.10101000.00000000.00000000