Assigning Private IP Network Numbers

Private IP Network Numbers

Private IP networks are "organizationally scoped" IP nets, i.e. networks which institutions can use internally but which are not routed outside of that organization. (Specifically: "...routing information about private networks shall not be propagated on inter-enterprise links, and packets with private source or destination addresses should not be forwarded across such links." - RFC1918 — Address Allocation for Private Internets.)

Assigning Private IP Network Numbers

RFC1918 does not explicitly define "enterprise"; it could be interpreted as "the University of Michgan", or as the UM Ann Arbor Campus (with the Flint and Dearborn campuses each being independent enterprises), or as the UMnet backbone (with the CAEN and UMHS backbones being independent enterprises), or one could consider each school/college/unit as a separate enterprise.

Historically, we have considered the Ann Arbor campus to be the "enterprise", and private IP address space on the Ann Arbor campus has been divided up between the three backbone providers: ITCom (UMnet), CAEN, and UMHS (see below). Each of these is responsible for administration of subnets within its assigned ranges. Ranges of private IP space designated as "shared" may be routed between the three groups—i.e. their scope is the entire U-M Ann Arbor campus. "Backbone-local" ranges will not be routed between the three Ann Arbor backbones.

In September 2003, one block of private IP space (172.31.0.0/17) was made routable between the UMnet backbone and the U-M Dearborn campus. At the same time another block (172.31.128.0/17) was set aside for possible future routing between the Ann Arbor campus and U-M Flint. In addition, the 10.215.0.0/16 and 10.216.0.0/16 blocks have been reserved for possible inter-campus routing with Dearborn and Flint, respectively (see below).

Units who wish to use private IP addresses on their networks must request address space from their on-campus backbone provider to ensure that addressing conflicts do not occur.

University of Michigan Private IP Network Assignments

*Note: 10.0.0.0/8 addresses which "mirror" the public address spaces currently in use — i.e. networks with prefixes of 10.194, 10.211, 10.212, 10.213, 10.214, 10.215, 10.216 and 10.75.<144-159> — are reserved and may be announced only by the appropriate owner. This includes one exception to the "Backbone-local" range (10.0.0.0/9) and several exceptions to the UMHS range (10.128.0.0/11). There is also one exception to the "must not be routed" range: 10.111.0.0/16 is announced by and routed to the ITCom Engineering iLab.

**Within UMnet, the 10.255.0.0/16 net is reserved for "LAN-local" use, i.e. it may be used freely on any LAN and will never be routed across the UMnet backbone.

*Within UMnet, 10.75.V, 10.108.W, 10.194.X, 10.211.Y and 10.213.Z subnets that mirror existing 207.75.v, 198.108.w, 67.194.X, 141.211.Y and 141.213.Z subnets are reserved for use on the networks where those public IP subnets reside; units needing private IP space can generally assume that these subnets will be available for intra-campus use. Please contact umnet-admin@umich.edu to arrange for the activation of these private subnets.

Within UMnet, the 192.168.255.0/24 net is reserved for "LAN-local" use, i.e. it may be used freely on any LAN and will never be routed across the UMnet backbone.

If these assignments need to be adjusted, UMnet, UMHS and CAEN must all be informed. The current contacts for the three campuses are:

For information on specific IP subnet assignments (public or private) or other information regarding networks on the UMnet backbone, please visit the Network Information Database page.

The UMnet Administration Team
umnet-admin@umich.edu