CIDR /17 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 /17 subnet.

IPv4 /17 Subnet Details

Medium-sized network with thousands of IP addresses. These are suitable for medium to large enterprises, allowing for a significant number of hosts or further subnetting.

IPv4 Subnet Information

CIDR Notation/17
Subnet Mask255.255.128.0
Binary Subnet Mask11111111 11111111 10000000 00000000
Total Addresses32,768
Usable Hosts (Standard)32,766
AWS Usable IPs32,761 (5 reserved)
Azure Usable IPs32,761 (5 reserved)
GCP Usable IPs32,762 (4 reserved)

IPv6 /17 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

/0/1/2/3/4/5/6/7/8/9/10/11/12/13/14/15/16/18/19/20/21/22/23/24/25/26/27/28/29/30/31/32