Subject: Re: Why "add net 244.0.0.0" at boot?

From: [email-masked] (Frank Peters)
Date: 5 Aug 1993 22:37:03 GMT

In article <[email-masked]> [email-masked] (EStrickler) says:
: When I boot my SparcLX running Solaris 2.2,
: I get a message:
: 
: setting netmask of le0
: setting default interface for multicast: add net 244.0.0.0: gateway name
: 
: Why and what is net 244.0.0.0? My network people are confused with
: why I have 244.0.0.0 in my routing tables.

Solaris 2 adds support for multicasts (see RFC 1112 for a protocol 
description and RFC 1301 for a good example of its uses).

Basically, multicast is sort of a directed broadcast. Every machine on
a network receives packets sent to the broadcast address.  Only the
individual machine receives packets sent to its specific IP address
(I'm ignoring promiscuous mode and sniffers and the like).

Suppose machine A wants to send a packet of information to machines B,
G and H but not to machines C, D, E and F.  With conventional IP it can
send three packets, one to each destination, or a broadcast that every
machine on the network will see.

With multicast the machines can agree on a multicast address to listen
to (I think of it as "subscribing" to the address).  Machine A can then
send its packet to that multicast address and all of the subscribed
machines will see it while none of the unsubscribed machines will.
This has a variety of potential uses...think of a group of machines
keeping a distributed database in sync or a network "radio broadcast"
system (you "tune in" to a station by joining the multicast address it
is sending to).

Multicast uses the class D IP address range (yes, there is a range of
addresses that aren't class A, B or C addresses) from 224.0.0.0 to
239.255.255.255.  And address 224.0.0.1 is the "permanent group of all
IP hosts" (from RFC 1112).

All of which is a long winded way of saying, its normal, harmless and
potentially useful if you ever have an application that takes advantage
of it.
--
Frank Peters  -  UNIX Systems Programmer  -  Mississippi State University
Internet: [email-masked]  -  Phone: (601)325-7030  -  FAX: (601)325-8921