Siemens SIMATIC NET SCALANCE S615 Configuration Manual page 32

Technical basics
3.1 Structure of an IPv4 address
Classless Inter-Domain Routing (CIDR)
CIDR is a method that groups several IPv4 addresses into an address range by representing
an IPv4 address combined with its subnet mask. To do this, a suffix is appended to the IPv4
address that specifies the number of bits of the network mask set to 1. Using the CIDR notation,
routing tables can be reduced in size and the available address ranges put to better use.
Example:
IPv4 address 192.168.0.0 with subnet mask 255.255.255.0
The network part of the address covers 3 x 8 bits in binary representation; in other words 24 bits.
This results in the CIDR notation 192.168.0.0/24.
The host part covers 1 x 8 bits in binary notation. This results in an address range of 2 to the
power 8, in other words 256 possible addresses.
Masking additional subnets
Using the subnet mask, you can further structure a subnet assigned to one of the address
classes A, B or C and form "private" subnets by setting further lower-level digits of the subnet
mask to "1". For each bit set to "1", the number of "private" networks doubles and the number
of nodes contained in them is halved. Externally, the network still looks like a single network.
Example:
You change the default subnet mask for a subnet of address class B (e.g. IP address
129.80.xxx.xxx) as follows:
Masks
Default subnet mask
Subnet mask
Result:
All devices with addresses from 129.80.001.xxx to 129.80.127.xxx are on one IP subnet, all
devices with addresses from 129.80.128.xxx to 129.80.255.xxx are on another IP subnet.
Network gateway (router)
The task of the network gateways (routers) is to connect the IP subnets. If an IP datagram is to
be sent to another network, it must first be sent to a router. For make this possible, you need
to enter the router address for each member of the IP subnet.
The IP address of a device in the subnet and the IP address of the network gateway (router)
may only be different at the points where the subnet mask is set to "0".
32
Decimal
255.255.0.0
255.255.128.0
Configuration Manual, 11/2019, C79000-G8976-C388-08
Binary
11111111.11111111.00000000.
00000000
11111111.11111111.10000000.
00000000
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