Class-full IP Addresses

Class-full IP Addresses

IP addresses are divided into classes A, B, C, D and E. Classes A, B and C are used for addressing end devices like computers, printers, routers etc.

Class D addresses are used for multicast and Class E addresses are reserved for experimental purposes.

Class A Addresses

Class A addresses have the first octet in the range 1-126. The first bit of a Class A address is always 0.

In binary, Class A addresses look like: 0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

The default subnet mask for Class A is 255.0.0.0 or /8

Examples of Class A networks:

• 10.0.0.0/8 - Private network
• 8.0.0.0/8 - Google's public DNS
• 127.0.0.0/8 - Loopback addresses

Class B Addresses

Class B addresses have the first octet in the range 128-191. The first two bits of a Class B address are always 10.

In binary, Class B addresses look like: 10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

The default subnet mask for Class B is 255.255.0.0 or /16

Examples of Class B networks:

• 172.16.0.0/16 - Private network range
• 169.254.0.0/16 - APIPA addresses

Class C Addresses

Class C addresses have the first octet in the range 192-223. The first three bits of a Class C address are always 110.

In binary, Class C addresses look like: 110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx

The default subnet mask for Class C is 255.255.255.0 or /24

Examples of Class C networks:

• 192.168.0.0/24 - Private network range
• 8.8.8.0/24 - Google's DNS servers

Special Address Ranges

There are several special address ranges to be aware of:

• Private IP Ranges (RFC 1918):
• Class A: 10.0.0.0/8
• Class B: 172.16.0.0/12 (172.16.0.0 - 172.31.255.255)
• Class C: 192.168.0.0/16 (192.168.0.0 - 192.168.255.255)
• Loopback: 127.0.0.0/8
• APIPA: 169.254.0.0/16
• Multicast: 224.0.0.0/4 (Class D)

Class-full vs Class-less

While class-full addressing is still important to understand, modern networks primarily use class-less addressing (CIDR - Classless Inter-Domain Routing). CIDR allows for more flexible subnet masks that don't follow the traditional class boundaries.

For example, instead of using the default /24 mask for a Class C network, you might use /25, /26, or /30 depending on your needs.