taap.digitalnetwork engineering toolkit

How many /48 subnets are in a /40?

Answer
256 /48 subnets in a /40

The difference between the prefix lengths is 48 − 40 = 8 bits, so a /40 holds 28 = 256 /48 subnets. A /40 is a common regional or POP aggregate inside an ISP's /32.

subnet6 · adjust the values

The math

2(48 − 40) = 28 = 256

The subnet ID lives in the last two hex digits of the third hextet. Each /48 still contains 65,536 /64 LAN segments.

First subnets of 2001:db8::/40

#/48 subnet
12001:db8::/48
22001:db8:1::/48
32001:db8:2::/48
42001:db8:3::/48
2562001:db8:ff::/48

Addresses are written in RFC 5952 form: lowercase, no leading zeros, longest zero run compressed to ::.

Planning tip

Keep subnet boundaries on nibbles (multiples of 4 bits) so each subnet ID maps to whole hex digits and reverse DNS (ip6.arpa) delegation stays clean. Encode structure (site, function, VLAN) in those digits rather than allocating sequentially.

More questions like this

/64 in a /48/64 in a /56/64 in a /60/64 in a /52/48 in a /32/48 in a /44/56 in a /48/64 in a /32

Frequently asked questions

How many addresses are in each /48?

280 addresses. That equals 65,536 /64 subnets.

How many /64s are in a /40?

16,777,216 (224).