blank.gif (43 bytes)

Church Of The
Swimming Elephant

7. Broadcast IP Addressing - Conventions Connected: An Internet Encyclopedia
7. Broadcast IP Addressing - Conventions

Up: Connected: An Internet Encyclopedia
Up: Requests For Comments
Up: RFC 922
Prev: 6.3. Pseudo-Algol Routing Algorithm
Next: 7.1. ARP Servers and Broadcasts

7. Broadcast IP Addressing - Conventions

7. Broadcast IP Addressing - Conventions

If different IP implementations are to be compatible, there must be convention distinguished number to denote "all hosts" and "all subnets".

Since the local network layer can always map an IP address into data link layer address, the choice of an IP "broadcast host number" is somewhat arbitrary. For simplicity, it should be one not likely to be assigned to a real host. The number whose bits are all ones has this property; this assignment was first proposed in [6]. In the few cases where a host has been assigned an address with a host-number part of all ones, it does not seem onerous to require renumbering.

The "all subnets" number is also all ones; this means that a host wishing to broadcast to all hosts on a remote IP network need not know how the destination address is divided up into subnet and host fields, or if it is even divided at all. For example, may denote all the hosts on a single hardware network, or all the hosts on a subnetted IP network with 1 byte of subnet field and 2 bytes of host field, or any other possible division.

The address denotes a broadcast on a local hardware network that must not be forwarded. This address may be used, for example, by hosts that do not know their network number and are asking some server for it.

Thus, a host on net 36, for example, may:

  • broadcast to all of its immediate neighbors by using

  • broadcast to all of net 36 by using

without knowing if the net is subnetted; if it is not, then both addresses have the same effect. A robust application might try the former address, and if no response is received, then try the latter. See [1] for a discussion of such "expanding ring search" techniques.

If the use of "all ones" in a field of an IP address means "broadcast", using "all zeros" could be viewed as meaning "unspecified". There is probably no reason for such addresses to appear anywhere but as the source address of an ICMP Information Request datagram. However, as a notational convention, we refer to networks (as opposed to hosts) by using addresses with zero fields. For example, means "network number 36" while means "all hosts on network number 36".

Next: 7.1. ARP Servers and Broadcasts

Connected: An Internet Encyclopedia
7. Broadcast IP Addressing - Conventions


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