Network and Broadcast address.

In every network, there is two addresses used for network and broadcast addressing. These addresses are not available for node addressing. So if there are 4 addresses in the network, there can be 2 nodes defined. Only exception is network with 32 bit mask (network with only one address), which is used for defining the nodes local IP address in the routing table. It means mask with 31 bits, network with 2 addresses, can not be used.

Network address is first address in the network and it is used for identification network segment. All the IP addresses, using the same network address part, are in the same network segment. Because network address is first address in the network, it can not be random IP address, but it must mach with network mask in a binary view, for last bits in the network address must be zeros, as long as mask has zeros.

As an example:

193.040.136.064 = 11000001.00101000.10001000.01000000 (network)

255.255.255.240 = 11111111.11111111.11111111.11110000 (mask)

In private networks, inside organizations, there is mostly used mask 255.255.255.0, because of simplicity of calculations of network address – last octet is zero, and first three octets are representing network address.

Broadcast address is the last address in the network, and it is used for addressing all the nodes in the network at the same time. It means that IP packet, where the destination address is broadcast address, is sent to all nodes of the IP network. It is important for remote announcements in network segment. In some cases it is used for attacking purposes by hackers or can cause problems in bigger network segments.

Read more:

http://en.wikipedia.org/wiki/Broadcast_address  http://en.wikipedia.org/wiki/Broadcast_traffic

http://en.wikipedia.org/wiki/Broadcast_storm

What is Gateway and how to calculate network and broadcast address, if you have node address and mask, is covered in the “IP routing” unit.