IPv6

Why IPv6?

IPv4's 4.3 billion addresses ran out: the regional internet registries handed out their last large IPv4 blocks between 2011 and 2019. IPv6 uses 128-bit addresses, about 3.4 × 10^38 of them, so every device can have a globally unique address and NAT is no longer needed for address sharing.

Writing IPv6 addresses

Eight groups of four hexadecimal digits, separated by colons:

2001:0db8:0000:0000:0000:ff00:0042:8329
2001:db8::ff00:42:8329        same address, shortened

Leading zeros in a group can be dropped, and one run of all-zero groups can be replaced with ::.

import ipaddress

ip = ipaddress.ip_address("2001:0db8:0000:0000:0000:ff00:0042:8329")
print("compressed:", ip.compressed)
print("exploded:  ", ip.exploded)
print("loopback:  ", ipaddress.ip_address("::1").is_loopback)

compressed: 2001:db8::ff00:42:8329
exploded:   2001:0db8:0000:0000:0000:ff00:0042:8329
loopback:   True

IPv6 in the cloud

• A typical LAN or cloud subnet is a /64 (18 quintillion addresses). AWS gives a VPC a /56 and each subnet a /64.
• IPv6 addresses are usually public, so "private" subnets use an egress-only internet gateway (AWS) or firewall rules to allow outbound connections only, instead of NAT.
• Dual-stack networks run IPv4 and IPv6 side by side; DNS AAAA records hold IPv6 addresses.

import ipaddress

vpc = ipaddress.ip_network("2001:db8:1200::/56")
subnets = vpc.subnets(new_prefix=64)
print(vpc.num_addresses == 2 ** 72, "- a /56 holds 2**72 addresses")
for _ in range(3):
    print("subnet:", next(subnets))
print("a /56 holds", 2 ** (64 - 56), "subnets of /64")

True - a /56 holds 2**72 addresses
subnet: 2001:db8:1200::/64
subnet: 2001:db8:1200:1::/64
subnet: 2001:db8:1200:2::/64
a /56 holds 256 subnets of /64