IPv6 /42 Prefix ReferenceIPv6
Nobody orders a /42 by name, yet it keeps showing up: quarter a /40 and four of these fall out, each carrying 4,194,304 /64 subnets. Think of it as the honest middle step between a provider's /40 and a tenant's /44.
/42 facts at a glance
- Total addresses
- 77,371,252,455,336,267,181,195,264 (286)
- /64 subnets inside
- 4,194,304
- Host/interface bits
- 86
- Example network
- 2001:db8::/42
- First address
- 2001:db8::
- Last address
- 2001:db8:3f:ffff:ffff:ffff:ffff:ffff
- Typical role
- Aggregation prefix between allocations
A /42 in documentation space
Using 2001:db8::/32 (RFC 3849 documentation space) aligned to its own boundary, a /42 occupies:
| Block | First address | Last address |
|---|---|---|
| 2001:db8::/42 | 2001:db8:: | 2001:db8:3f:ffff:ffff:ffff:ffff:ffff |
How an IPv6 /42 prefix works
A /42 prefix pins the first 42 bits of a 128-bit address, leaving 86 bits free. That free portion holds 286 = 77,371,252,455,336,267,181,195,264 distinct addresses, and every such block begins at a multiple of that size. Because each subnet should be a /64, the 22 middle bits between the prefix and the /64 boundary give 4,194,304 subnets to hand out.
Where /42 is used
- Quartering a /40 into four chunks, each ready to split further into /44s.
- Intermediate staging during top-down subdivision of larger aggregates.
- Lab and training environments practicing hierarchical delegation math.
The counts stay friendly: one /42 equals four /44s or sixty-four /48s.
Because /42 misses the nibble grid, it almost always plays the role of internal scaffolding rather than a formally issued grant.
Frequently asked questions
How many /44s are in a /42?
Four. Two bits of difference, so halving twice takes you from /42 down to /44.
How many /48s fit in a /42?
Sixty-four — six bits' worth of classic site allocations.
How many addresses does an IPv6 /42 have?
77,371,252,455,336,267,181,195,264, which is 2^86.
Open this prefix in the calculator
Open 2001:db8::/42 in the IPv6 calculator