function
- on the same subnet, host can send traffic directly to each other
- to send traffic to a host on a different subnet, it must be forwarded by a router
- the
subnet mask
- enables the host to understand, if the destination is on the same or a different subnet
- 32 bits long
- written in dotted or slash notation
network and host portion
- host’s IP address consists of:
- network portion and host portion
- the subnet mask defines the boundary between both portions
example
- host’s IP address: 192.168.10.15
- subnet mask: 255.255.255.0
host IP in binary
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 1 (64) |
1 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 1 (40) |
0 (40) |
1 (8) |
0 (8) |
1 (0) |
0 (0) |
0 (0) |
0 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 0 (10) |
0 (10) |
0 (10) |
0 (10) |
1 (2) |
0 (2) |
1 (0) |
0 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 0 (10) |
0 (10) |
0 (10) |
0 (10) |
1 (7) |
1 (3) |
1 (1) |
1 (0) |
subnet mask in binary
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 1 (127) |
1 (63) |
1 (31) |
1 (15) |
1 (7) |
1 (3) |
1 (1) |
1 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 1 (127) |
1 (63) |
1 (31) |
1 (15) |
1 (7) |
1 (3) |
1 (1) |
1 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 1 (127) |
1 (63) |
1 (31) |
1 (15) |
1 (7) |
1 (3) |
1 (1) |
1 (0) |
| 128 |
64 |
32 |
16 |
8 |
4 |
2 |
1 |
| 0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
0 (0) |
interpretation
- IP address is compared with the subnet mask
1in the subnet mask: that bit in the IP address is part of the network address0in the subnet mask: the bit is part of the host address- network address portion: 192.168.10
- host address portion: 15
consequences
- host wants to communicate with 192.168.10.20
- same subnet –> traffic canbe sent directly
- host wants to communicate with 192.168.11.20
- traffic has to be sent via router
valid subnet masks
- always begins with contiguous array of 1s#
- valid: 11111111.11110000.00000000.00000000
- invalid: 11111101.11110000.00000000.00000000
host portion
- the
host portion of the address is allocated to the different hosts on the subnet (e.g. PCs, servers, printers, router, interfaces, swithc management addresses)
- two exceptions:
- all 0 in the host portion designates the network address
- in our example, the network address is: 192.168.10.0
- all 1 in the host portion designates the directed broadcast address for the subnet
- traffic with this destination will be sent to all hosts in the subnet
- in our example, the broadcast address is: 192.168.10.255
- host portion of the address must be unique on the subnet
- no need for sequential numbering
- having a host with 10.10.10.10 and another with 10.10.10.20 is valid
- this leaves us with 254 possible host addresses in this subnet
network address (network ID)
- the array of 0s in the subnet mask signifies the network address
- all 0s in the host portion is reserved for the network address
- in our example, the network address is: 192.168.10.0
slash notation
- subnet mask always begins with contiguous 1s
- this array is thus 1 to 32 bits long (from left to right)
- this allows for slash notation
- 255.255.255.0 is equal to /24 (which signifies 24 contiguous 1s)
- 255.255.0.0 is equal to /16 (which signifies 16 contiguous 1s)
- used in conversations and on network diagrams
- example 1:
- 192.168.10.15 255.255.255.0 or
- 192.168.10.15/24
- the network address is 192.168.10.0/24
- example 2:
- 10.10.10.15 255.0.0.0 or
- 10.10.10.15/8
- the network address is 10.0.0.0/8