Class A Networks

The leading bit of a Class A address is always 0. This allocation leaves 7 bits for the network portion and 24 bits for the host portion.

• First Octet Range: 1 to 126 (00000001 to 01111111)
• Default Subnet Mask: 255.0.0.0 (/8)
• Host Capacity: 2^24 - 2 = 16,777,214 usable hosts per network (subtracting the network identifier and the directed broadcast address).

Reserved Ranges:

• 127.0.0.0/8: Designated for loopback testing and internal diagnostics.
• 10.0.0.0/8: Private address space for internal local area networks (not routable on the public internet).

Class B Networks

Class B addresses begin with the binary prefix 10. The architecture allocates 14 bits to the network and 16 bits to the host.

• First Octet Range: 128 to 191 (10000000 to 10111111)
• Default Subnet Mask: 255.255.0.0 (/16)
• Host Capacity: 2^16 - 2 = 65,534 usable hosts per network.

Reserved Ranges:

• 169.254.0.0/16: Allocated for Automatic Private IP Addressing (APIPA), utilized when a host fails to obtain an IP via DHCP.
• 172.16.0.0/12: Private address block spanning from 172.16.0.0 to 172.31.255.255, restricted to internal network usage.

Class C Networks

Identified by the initial bits 110, Class C designates 21 bits to the network and 8 bits to the host.

• First Octet Range: 192 to 223 (11000000 to 11011111)
• Default Subnet Mask: 255.255.255.0 (/24)
• Host Capacity: 2^8 - 2 = 254 usable hosts per network.

Reserved Range:

• 192.168.0.0/16: Private address space for local networks (192.168.0.0 to 192.168.255.255).

Class D Networks

Class D is defined by the high-order bits 1110. Unlike Classes A, B, and C, it does not utilize traditional host/network subdivisions.

• First Octet Range: 224 to 239 (11100000 to 11101111)
• Purpose: Reserved exclusively for multicast group addressing rather than individual host assignment.

Subnet Mask Functionality

A subnet mask specifies which portion of an IP address represents the network and which portion represents the host. By applying a bitwise AND operation between the IP address and the subnet mask, devices determine if a destination IP resides on the same local subnet or requires routing to a different network.

For instance, 10.1.2.3/8 and 10.5.6.7/8 share the same network identifier due to the 255.0.0.0 mask, confirming they are on the same logical segment.

Private Address Space Summary

Private IP blocks are strictly designated for internal network communications and are blocked from routing over the public internet.

• Class A: 10.0.0.0/8
• Class B: 172.16.0.0/12
• Class C: 192.168.0.0/16

IPv4 Class Summary