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From: rbj@icst-cmr.arpa (Root Boy Jim)
Newsgroups: comp.unix.wizards
Subject: Trouble with Ethernet
Message-ID: <9473@brl-adm.ARPA>
Date: Thu, 24-Sep-87 12:47:10 EDT
Article-I.D.: brl-adm.9473
Posted: Thu Sep 24 12:47:10 1987
Date-Received: Sat, 26-Sep-87 17:06:44 EDT
Sender: news@brl-adm.ARPA
Lines: 127


   It appears that the problem is that the Vax (and the PCs) have internet
   addresses 126.1.0.*, whereas the Sun (and its client) have addresses
   192.9.200.*.  (These aren't properly assigned internet addresses, but we
   aren't connected to the internet.)  It appears that both the Sun and Vax
   believe that all hosts on a single Ethernet have the first three parts of
   their addresses the same.  Thus, the Sun refuses to put packets on the net
   for addresses other than 192.9.200.*, and the Vax refuses to put packets
   on the net for addresses other than 126.1.0.*.  Thus, the Sun and Vax
   can't talk to each other, and a PC can talk to only one of the two at a
   time (depending on the address that the PC is set to respond to).

   There seem to be two possible solutions:

   1. Change the addresses of the Sun (and its client diskless Sun) to
   126.1.0.* addresses.

   2. Get the Sun and the Vax to understand that both sets of addresses are
   physically on the same Ethernet.  (Since they are both BSD 4.2 versions,
   the same fix should work for both.)

Well, that reminds me of a war story. We went from a Class C net
(192.12.119.xx) to a Class B net (129.6.xx.xx), subnet 48.

We have Vaxen running 4.2 and 4.3, a Sequent running Dynix V2.0.1, and
various Sun-2's and Sun-3's running anything from 2.0 to 3.2.

Only 4.3 has the `netmask' parameter, altho I think it appeared in Sunix 3.4.

Everything seemed to work, except the 4.2 Vaxen. My first clue was when
netstat -r left off the chars enclosed in <<<>>> below:

Routing tables
Destination          Gateway              Flags    Refcnt Use        Interface
127.0.0.1            127.0.0.1            UH       1      157        lo0
129.6.48.154         127.0.0.1            UH       0      3655       lo0
default              129.6.48.1           UG       0      10781      il0
129.6<<<.48>>>       129.6.48.154         U        2      151992     il0

I figured it had something to do with Marxism (class struggle :-), and
found the specific code, in /sys/netinet/in.c. Fortunately, all we had to
do was #ifdef out an `if' clause of two. Our changes are only the `#' lines.

#define FORCE_CLASS_C 1
/*
 * Formulate an Internet address from network + host.  Used in
 * building addresses stored in the ifnet structure.
 */
struct in_addr
if_makeaddr(net, host)
	int net, host;
{
	u_long addr;
	if (net < 128)
		addr = (net << IN_CLASSA_NSHIFT) | host;
#ifndef FORCE_CLASS_C
	else if (net < 65536)
		addr = (net << IN_CLASSB_NSHIFT) | host;
#endif
	else
		addr = (net << IN_CLASSC_NSHIFT) | host;
	addr = htonl(addr);
	return (*(struct in_addr *)&addr);
}

/*
 * Return the network number from an internet address.
 */
in_netof(in)
	struct in_addr in;
{
	register u_long i = ntohl(in.s_addr);

	if (IN_CLASSA(i))
		return (((i)&IN_CLASSA_NET) >> IN_CLASSA_NSHIFT);
#ifndef FORCE_CLASS_C
	else if (IN_CLASSB(i))
		return (((i)&IN_CLASSB_NET) >> IN_CLASSB_NSHIFT);
#endif
	else
		return (((i)&IN_CLASSC_NET) >> IN_CLASSC_NSHIFT);
}

/*
 * Return the host portion of an internet address.
 */
in_lnaof(in)
	struct in_addr in;
{
	register u_long i = ntohl(in.s_addr);
	if (IN_CLASSA(i))
		return ((i)&IN_CLASSA_HOST);
#ifndef FORCE_CLASS_C
	else if (IN_CLASSB(i))
		return ((i)&IN_CLASSB_HOST);
#endif
	else
		return ((i)&IN_CLASSC_HOST);
}

Originally, we tried always forcing Class C, but it turns out there *is*
one Class A address we're interested in: 127.0.0.1, AKA localhost.

Interestingly enuf, it seems that Sunix and Dynix do this also (we don't
have Sun source; we do have Dynix, but it's offline so I can't check).
Thus, these systems DO THE RIGHT THING FOR THE WRONG REASON!!! They
seem to be FORCING CLASS C MODE!!! The reason it's the right thing to do
is that people seem to run on subnets anyway. I don't know what the
gateway does, or how it's set up, but it's a Vax 750 running 4.3.

But to get back to the original poster's question, my recommendation
would be to run your network on a Class C net. That is, the first
number should be 192 or higher, change the Vax and the PC's to use
the same network number as the Suns. Comments anyone?

Apologys to those of you who have to see this twice, but I wanted a wider
audience. Special apologys to Chris and my bro, who have to see it three
times, and to Bill Majurski (also of NBS), who had to live thru it.

   Dale Worley     Cullinet Software
   ARPA: cullvax!drw@eddie.mit.edu
   UUCP: ...!seismo!harvard!mit-eddie!cullvax!drw

	(Root Boy) Jim Cottrell	<rbj@icst-cmr.arpa>
	National Bureau of Standards
	Flamer's Hotline: (301) 975-5688
	I'll show you MY telex number if you show me YOURS...