Technical data

www.westermo.com Theoretical and general applications 117
The following standard netmasks (i.e. a without subnetwork) apply to the address
classes A, B and C:
As described earlier, a Class B IP address consists of two equal sized address parts,
2 bytes each for the network and device identity, this can be written N.N.D.D, where
N represents the octet belonging to the network identity and D the device identity,
whereby the netmask becomes 255.255.0.0.
If the full 3rd octet is used to define the subnetwork instead of a device identity, the
address can be interpreted as N.N.N.E, i.e. the netmask becomes 255.255.255.0.
This means we have 254 C-like networks with 254 computers in each (first and last
addresses in the network and computer parts are reserved).
In principle any of the bits in an octet can be used to define a subnetwork, normally
the highest bits are reserved for this, as it significantly simplifies management.
If, for example, the first three bits in a C address are used for subnetwork addresses,
the C network would be divided into 6 subnetworks (see the possible combinations
of networks as set out below). Two bit combinations of the device identity (11111 and
00000) are reserved for broadcast and network identity, which is why the number of
available addresses will be 30 on each of these networks.
Address Netmask Binary value Binary value Binary value Binary value
class Byte 1 Byte 2 Byte 3 Byte 4
A 255.0.0.0 11111111 00000000 00000000 00000000
B 255.255.0.0 11111111 11111111 00000000 00000000
C 255.255.255.0 11111111 11111111 11111111 00000000
Netmask C-like 3 first bits Other bits Subnet Number of
netmask in the C-like in the C-like work device
netmask netmask identities
255.255.32.0 32 001 00000 1 30
255.255.64.0 64 010 00000 2 30
255.255.96.0 96 011 00000 3 30
255.255.128.0 128 100 00000 4 30
255.255.160.0 160 101 00000 5 30
255.255.192.0 192 110 00000 6 30
B A C K