Subnet mask?

Question

If an IP network 192.168.130.0 uses the subnet mask 255.255.255.224, why can't 192.168.130.250 be used as a host address?

Answer 1

This is because you're subnetting the last octet.

255.255.255.224 is a /27 submask. This means that you have several networks within the 192.168.130.0-192.168.130.255 range. With a /27 subnet, you have eight networks.

192.168.130.0
192.168.130.32
192.168.130.64
192.168.130.96
192.168.130.128
192.168.130.160
192.168.130.192
192.168.130.224

This means, assuming that all your other hosts are placed on 192.168.130.1 and upwards, that it's most likely on a network different from the one you're trying to connect from.

Answer 2

The subnet mask is used by the PC to determine which computers belong to which network. This may get a little technical but please bear with me.

The PC does a logical AND with the PC's IP address and the subnet mask to get the base address of the "network".

If two PC's have the same "base" address, then they are on the same network.

So, 192.168.130.1 and 255.255.255.224 look like this in Binary and get this result.

11000000. 10101000. 10000010. 00000001
11111111. 11111111. 11111111. 11100000
==============================
11000000. 10101000. 10000010. 00000000

or 192.168.130.0. that's the base network as you said.

192.168.130.250 AND 255.255.255.224 would achieve this result...

11000000. 10101000. 10000010. 11111010
11111111. 11111111. 11111111. 11100000
==============================
11000000. 10101000. 10000010. 11100000

or 192.168.130.224 as a base network address.

Since 192.168.130.0 and 192.168.130.224 are different base addresses, they are not on the same network.

Anything in the last octet (the 224) of the subnet mask means it is subnetted. It means it is set up as 8 networks. The 130.250 is a different network than the 130.0

Answer 3

Example:
Subnet is: 255.255.252.0
in binary: 11111111.11111111.11111100.00000000

The "0's" are reserved for the host, while the "1's" represent the network.

So 192.168.100.x to 192.168.103.x belong to the same network.
Look at the 3rd octet (100 - 103)
100: 1100100
101: 1100101
102: 1100110
103: 1100111
While 192.168.104.x to 192.168.107.x is a different network.
104: 1101000
105: 1101001
106: 1101010
107: 1101011

HTH