blank.gif (43 bytes)

Church Of The
Swimming Elephant

Reserved Subnet Numbers Connected: An Internet Encyclopedia
Reserved Subnet Numbers

Up: Connected: An Internet Encyclopedia
Up: Programmed Instruction Course
Up: Subnetting and CIDR
Prev: A Simple Subnet
Next: Subnetting

Reserved Subnet Numbers For largely historical reasons, two addresses are reserved on every subnet. They are the smallest and largest addresses - those two with all 0s and all 1s in the host field (the bits to the right of the prefix boundary). No host can be assigned either of these reserved addresses. For example, a subnet assigned the prefix would have and as its reserved addresses.

The all 0s address was used by older routing protocols to distinguish a subnet route from a 32-bit host route. The all 1s address was used to broadcast to all hosts on the subnet. Since newer routing protocols are fully capable of handling routing prefixes of any length, and anything that can be done with broadcast can be done better with multicast, both of these reserved addresses belong in the arcana of Internet history books. Unfortunately, some TCP/IP implementations still use these reserved addresses, and many more implementations maintain backwards compatibility, so it's best to err on the safe side and not assign these two addresses.

A 32-bit prefix would be useless as a subnet prefix, since it only matches a single address, and what use is a cable with only one connection? A 31-bit prefix matches two address, but both of them would be reserved, so 31-bit prefixes aren't used either. A 30-bit prefix, matching four address, two of which can be assigned, is the longest usable subnet prefix. Of course, a 30-bit subnet can only have two hosts, so these prefixes are popular for use with serial links. Likewise, a 29-bit prefix matches eight addresses, of which six can be assigned. The most important prefix lengths, along with the number of assignable addresses, are shown in the table.

Prefix LengthMatching AddressesAssignable Addresses

The table could be extended to /15 and beyond, but there is very little use for a single subnet with even a /16 prefix - it's simply too large. Putting 65,000 devices on a single Ethernet (or other technology) just isn't practical.

Next: Subnetting

Connected: An Internet Encyclopedia
Reserved Subnet Numbers


Protect yourself from cyberstalkers, identity thieves, and those who would snoop on you.
Stop spam from invading your inbox without losing the mail you want. We give you more control over your e-mail than any other service.
Block popups, ads, and malicious scripts while you surf the net through our anonymous proxies.
Participate in Usenet, host your web files, easily send anonymous messages, and more, much more.
All private, all encrypted, all secure, all in an easy to use service, and all for only $5.95 a month!

Service Details

Have you gone to church today?
All pages ©1999, 2000, 2001, 2002, 2003 Church of the Swimming Elephant unless otherwise stated
Church of the Swimming Elephant©1999, 2000, 2001, 2002, 2003 is a wholly owned subsidiary of Packetderm, LLC.

Packetderm, LLC
210 Park Ave #308
Worcester, MA 01609