Subnet and IPv4 Classes
Last updated on May 17, 2023 pm
Subnet & IPv4 Classes
Subnet & Subnet Mask
For a random IP address a.b.c.d/x, x denotes the number of 1s in the subnet mask, starting from the most significant bits.
Divided by the subnet mask, the whole IP address can be seen as two parts: the network prefix and the host identifier.
For example, 192.168.19.123/20 is:
| Address Type | Decimal | Binary |
|---|---|---|
| IP Address | 192.168.19.123 |
11000000.10101000.00010011.01111011 |
| Subnet Mask | 255.255.240.0 |
11111111.11111111.11110000.00000000 |
| Host Identifier | 0.0.3.123 |
00000000.00000000.00000011.01111011 |
| Network Prefix | 192.168.16.0 |
11000000.10101000.00010000.00000000 |
| First Available Address | 192.168.16.1 |
11000000.10101000.00010000.00000001 |
| Last Available Address | 192.168.31.254 |
11000000.10101000.00011111.11111110 |
| Broadcast Address | 192.168.31.255 |
11000000.10101000.00011111.11111111 |
The subnet is always referred as its network prefix. It is used for routing outside the subnet.
Inside the subnet, connection and communication only care about the host identifier.
This two-layer separation makes routing more efficient and convenient.
IPv4 Classes
It’s wroth to note that IPv4 classes are already deprecated.
| Class | Decimal | Binary |
|---|---|---|
| A Start | 1.0.0.0 |
00000001.00000000.00000000.00000000 |
| A Private Start | 10.0.0.0 |
00001010.00000000.00000000.00000000 |
| A Private End | 10.255.255.255 |
00001010.11111111.11111111.11111111 |
| A End | 127.0.0.0 |
01111111.00000000.00000000.00000000 |
| A Mask | 255.0.0.0 |
11111111.00000000.00000000.00000000 |
| B Start | 128.0.0.0 |
10000000.00000000.00000000.00000000 |
| B Private Start | 172.16.0.0 |
10101100.00010000.00000000.00000000 |
| B Private End | 172.31.255.255 |
10101100.00011111.11111111.11111111 |
| B End | 191.255.0.0 |
10111111.11111111.00000000.00000000 |
| B Mask | 255.255.0.0 |
11111111.11111111.00000000.00000000 |
| C Start | 192.0.0.0 |
11000000.00000000.00000000.00000000 |
| C Private Start | 192.168.0.0 |
11000000.10101000.00000000.00000000 |
| C Private End | 192.168.255.255 |
11000000.10101000.11111111.11111111 |
| C End | 223.255.255.0 |
11011111.11111111.11111111.00000000 |
| C Mask | 255.255.255.0 |
11111111.11111111.11111111.00000000 |
| D Start | 224.0.0.0 |
11100000.00000000.00000000.00000000 |
| D End | 239.255.255.255 |
11101111.11111111.11111111.11111111 |
| E Start | 240.0.0.0 |
11110000.00000000.00000000.00000000 |
| E End | 255.255.255.255 |
11111111.11111111.11111111.11111111 |
Quote
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