ELSA Cable manual Examples, Same IP address, this time with another netmask

Technical basics 53

IP address

Network address

Netmask

the actual information to be transmitted (useful data), the data packet also contains address and control information.

IP addresses are used in IP networks for communications between various devices. In this case, every host has its own unique address by which it can be identified unambiguously. What does an IP address look like? It consists of four bytes separated by dots, making a total of 32 bits. Each of the four bytes can take on values from 0 to 255, e.g. 192.168.130.124.

To be precise, the IP address refers to the interface, and not the host itself. A terminal device with more than one interface (such as a router) has to have an IP address at its disposal for every single interface. This is why ISDN routers from ELSA for example, have an IP address for communication with the hosts in their own network, as well as a second IP address for communication with the “outside world” using the ISDN. In the same way, ELSA cable modems have an IP address for their own network and another IP address for the exchange of data with the cable network.

An IP address contains the address of the network as well as that of the host. The network address is the same for all hosts on one network, whereas the address of the host is exclusive and unique to the network. A router, for example, can have more than one IP address, each one unique to the network.

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:

This address...	...in bytes...	...looks like this in bits:
IP address	192.168.120.253	11000000.10101000.01111000.11111101
Netmask	255.255.255.0	11111111.11111111.11111111.00000000
Network address	192.168.120.0	11000000.10101000.01111000.00000000

The same IP address, this time with another netmask:

This address...	...in bytes...	...looks like this in bits:
IP address	192.168.120.253	11000000.10101000.01111000.11111101
Netmask	255.255.0.0	11111111.11111111.00000000.00000000
Network address	192.168.0.0	11000000.10101000.00000000.00000000

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.

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