Cambridge G-RIN (Geeks' Registry of Internet Numbers)
BCP5 (RFC1918) network numbers registry


BCP5 (currently RFC1918, `Address Allocation for Private Internets') describes circumstances where it can be useful to have non-globally-unique IP addresses, and specifies three IPv4 address ranges which may be used for this purpose. RFC1918 recommends that, to minimise the risk of clashes when private internets need to be merged, address ranges should be chosen at random.

This web page and database exist to help people choose ranges at random, and to help them choose ranges which are not currently in use by anyone else.

Pick a random subnet

Choose network size:
28 bit prefix (4 bit subnet, up to 14 hosts)
24 bit prefix (8 bit subnet, up to 254 hosts)
bit prefix.
bit subnet.

From which private-use range ? - recommended

Register a network range you are using

You may register an existing network range you are using in our database.

You must enter an email address to `own' the database entry. This email address will be mailed a reminder asking you to confirm/renew your entry periodically, and will also be used to authorise changes to the entry. It will be published on these pages unless you select to hide it; apart from that we will only use it to contact you about your database entry.

Network range:
Network name:
Contact name:
Email address:
Hide email address from public database.

View database

Cambridge G-RIN
Queries or problems ? Contact the administrator, Ian Jackson.