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ITS Networking & Telecommunications

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.)

Organizationally-Scoped (Private) IP Address Space

Class Starting Address Ending Address CIDR Notation Comments
A 10.0.0.0 10.255.255.255 10/8 16,777,216 addresses
B 172.16.0.0 172.31.255.255 172.16/12 1,048,576 addresses
C 192.168.0.0 192.168.255.255 192.168/16 65,536 addresses

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 UM-Ann Arbor campus. "Backbone-local" ranges will not be routed between the 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 UM-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 UM-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

Table 1: Campus Backbone Class A Private IP Assignments

Backbone Starting Addr Ending Address CIDR notation Comments
Any 10.0.0.0 10.127.255.255 10.0.0.0/9 Must not be routed between backbones*
UMHS 10.128.0.0 10.159.255.255 10.128.0.0/11 May be announced by UMHS
CAEN 10.160.0.0 10.191.255.255 10.160.0.0/11 May be announced by CAEN
Shared 10.192.0.0 10.223.255.255 10.192.0.0/11 Shared, announcements by mutual agreement
UMnet 10.224.0.0 10.255.255.255 10.224.0.0/11 May be announced by UMnet**

*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.

Table 2: Class A Private IP Exceptions

CIDR block Mirrors Announced by
10.75.144.0/20 207.75.144.0/20 UMnet*
10.108.8.0/21 198.108.8.0/21 UMnet*
10.111.0.0/16 198.111.224.0/22 ITCom Eng Lab
10.194.0.0/16 67.194.0.0/16 UMnet*
10.211.0.0/16 141.211.0.0/16 UMnet*
10.212.0.0/16 141.212.0.0/16 CAEN
10.213.0.0/17 141.213.0.0/17 CAEN
10.213.128.0/17 141.213.128.0/17 UMnet*
10.214.0.0/16 141.214.0.0/16 UMHS
10.215.0.0/16 141.215.0.0/16 Reserved for UM-Dearborn
10.216.0.0/16 141.216.0.0/16 Reserved for UM-Flint
10.230.32.0/19   CAEN (Plant Operations VLAN 20 on North Campus)
10.255.0.0/16 LAN-Local, not routed

*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.

Table 3: Campus Backbone Class B Private IP Assignments

Backbone Starting Addr Ending Address CIDR notation Comments
UMHS 172.16.0.0 172.16.255.255 172.16.0.0/16 May be announced by UMHS
UHMS 172.17.0.0 172.17.255.255 172.17.0.0/16 May be announced by UMHS
RESERVED 172.18.0.0 172.18.255.255 172.18.0.0/16 Reserved for future use
RESERVED 172.19.0.0 172.19.255.255 172.19.0.0/16 Reserved for future use
ANY 172.20.0.0 172.20.255.255 172.20.0.0/16 Must not be routed between backbones
CAEN 172.21.0.0 172.21.255.255 172.21.0.0/16 May be announced by CAEN
ANY 172.22.0.0 172.22.255.255 172.22.0.0/16 Must not be routed between backbones
UMnet 172.23.0.0 172.23.255.255 172.23.0.0/16 May be announced by UMnet
ANY 172.24.0.0 172.24.255.255 172.24.0.0/16 Must not be routed between backbones
ANY 172.25.0.0 172.25.255.255 172.25.0.0/16 Must not be routed between backbones
ANY 172.26.0.0 172.26.255.255 172.26.0.0/16 Must not be routed between backbones
RESERVED 172.27.0.0 172.27.255.255 172.27.0.0/16 Reserved for future use
VoIP 172.28.0.0 172.28.255.255 172.28.0.0/16 VoIP, may be announced by UMnet
VoIP 172.29.0.0 172.29.255.255 172.29.0.0/16 VoIP, may be announced by CAEN
VoIP 172.30.0.0 172.30.255.255 172.30.0.0/16 VoIP, may be announced by UMHS
VoIP 172.31.0.0 172.31.127.255 172.31.0.0/17 VoIP, reserved for UM-Dearborn
VoIP 172.31.128.0 172.31.255.255 172.31.128.0/17 VoIP, reserved for UM-Flint

Table 4: Campus Backbone Class C Private IP Assignments

Backbone Starting Addr Ending Address CIDR notation Comments
Shared 192.168.0.0 192.168.63.255 192.168.0/18 Shared, announcements by mutual agreement
Any 192.168.64.0 192.168.255.255 192.168.64/18
192.168.128.0/17
Must not be routed between backbones

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:

Campus Contact(s) Email Address
Medical Campus UMHS Networking MCIT-OTS-Engineering@med.umich.edu
Engineering CAEN Networking network-admin@engin.umich.edu
All Others UMnet Administration umnet-admin@umich.edu

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