CIDR /8 IPv4 & IPv6 Reference

Understanding CIDR notation for IPv4 and IPv6 subnets

CIDR (Classless Inter-Domain Routing) notation is a compact method for specifying IP addresses and their routing suffix. This page provides detailed information about the /8 subnet.

IPv4 /8 Subnet Details

Class A network with over 16 million hosts. Such large address blocks are typically managed by ISPs or very large corporations and are often subdivided extensively.

IPv4 Subnet Information

CIDR Notation/8
Subnet Mask255.0.0.0
Binary Subnet Mask11111111 00000000 00000000 00000000
Total Addresses16,777,216
Usable Hosts (Standard)16,777,214
AWS Usable IPs16,777,209 (5 reserved)
Azure Usable IPs16,777,209 (5 reserved)
GCP Usable IPs16,777,210 (4 reserved)

IPv6 /8 Subnet Details

Large allocation typically used by regional registries or major ISPs

IPv6 uses a 128-bit address space, a massive increase from IPv4's 32 bits. This allows for approximately 340 undecillion (3.4 x 1038) unique addresses, effectively eliminating the address exhaustion concerns of IPv4.

IPv6 addresses are represented as eight groups of four hexadecimal digits, separated by colons (e.g., 2001:0db8:85a3:0000:0000:8a2e:0370:7334). Leading zeros in each group can be omitted, and one sequence of consecutive all-zero groups can be replaced by a double colon (::), for instance 2001:db8::1.

Key Differences from IPv4 Subnetting

  • No Broadcast Addresses: IPv6 uses multicast for one-to-many communication instead of broadcast.
  • No Network/Broadcast Reserved Addresses: Unlike IPv4, the first and last addresses in an IPv6 subnet are typically usable host addresses. There's no concept of reserving addresses for network ID or broadcast.
  • Simplified Addressing: The fixed /64 boundary for host interfaces simplifies network design and enables SLAAC.
  • Header Simplicity: The IPv6 header is simpler than IPv4's, which can improve routing efficiency.

Other CIDR References

IPv4 CIDR

/0/1/2/3/4/5/6/7/9/10/11/12/13/14/15/16/17/18/19/20/21/22/23/24/25/26/27/28/29/30/31/32