RFC1918 describes a set of network ranges set aside for so-called "private" use. This document provides important information for IT Officers who may be considering using these addresses on their network.

Hostmaster at OUCS follows a policy of always providing public IP addresses to units where there is a clear need. The University is in a fortunate position in having considerable public IP space, bit it must be well-managed. For example devices which have no need to contact the Internet or be accessible to other systems in Oxford, such as printers and network switches, may be allocated RFC1918 addresses.

Where there is no requirement to use your public IP addresses allocated by Hostmaster at OUCS, you may be able to make use of these networks. For example, some devices which do not (and should not) need Internet access, such as IP CCTV cameras, may be good candidates for moving onto RFC1918.

The RFC1918 ranges are:

  • - (10/8 prefix)
  • - (172.16/12 prefix)
  • - (192.168/16 prefix)

The Networks team in OUCS has a requirement for large-scale IP allocation to devices which do not require Internet access. An example of this might be a VoIP telephone, although there are many others. We have elected to use the 172.16/12 address space for all OUCS Network infrastructure, and try to avoid use of the remaining RFC1918 ranges.

It is therefore recommended that IT Officers avoid use of 172.16/12 wherever possible. Although these IP addresses will not "leak" out of a unit's network onto the University backbone, there may be side-effects of using this range. For instance a number of them are registered in the DNS for management purposes, and you may find local devices believing their hostname to be something unexpected, depending on local DNS configuration.

In addition to the above, other subnets such as 10/8 are in use by OUCS but not in a way which could make them visible to another department or college in the University. Nor will these addresses be registered in the DNS within the University.

However, there should not be any significant negative side-effect if RFC1918 addresses are already in use, and this document is not a policy enforcing avoidance of this IP range, as other institutions such as Cambridge have.

The recommended RFC1918 subnets, namely 10/8 and 192.168/16, provide for over 16 million addresses. Hopefully that means those in use by OUCS won't be missed.