----- Original Message ----- From: "Willie Viljoen" <[EMAIL PROTECTED]> To: "Mark" <[EMAIL PROTECTED]> Cc: <[EMAIL PROTECTED]> Sent: Friday, February 21, 2003 7:24 PM Subject: Netmasks for aliases (was Re: Bizarre Networking Problem)
> On Friday 21 February 2003 19:17, Mark wrote: > > > > /sbin/ifconfig xl0 192.246.38.10 netmask 255.255.255.0 > > > /sbin/ifconfig xl0 alias 208.23.240.10 netmask 255.255.255.0 > > > > Hmm, I thought aliases always needed to have netmask of 255.255.255.255. > > Has something changed? > > > > - Mark > > Mark, > > Aliases on the same subnet should always be 255.255.255.255, this does not > apply where they are on different subnets, however. Here is a good > example, from the ifconfig output of a mail server I set up at an ISP in a > nearby town. They used to use a private range, but have since added a > public class C which has been broken up into smaller ranges for routing. > > The mail server, for legacy reasons, still has to serve people on the old > IPs because those are still being used by machines in their office, and > reconfiguring every office machine with new server IP addresses (they > didn't have internal DNS then) would be wasting time. We didn't want to > waste public IPs on beancounters, so we just left their machines as is. > > The host is also serving on the new IP, and acting as a POP server for > several virtual domains, which have aliases. The 10.0.1.0/24 range (their > office), and the 196.38.113.0/27 range (used for their server farm + > virtual domains) are still the same physical network though. The output > from ifconfig follows: > > %ifconfig > dc0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500 > inet 196.38.113.2 netmask 0xffffffe0 broadcast 196.38.113.31 > inet6 fe80::a00:8ff:fe00:800%dc0 prefixlen 64 scopeid 0x1 > inet 196.38.113.3 netmask 0xffffffff broadcast 196.38.113.3 > inet 196.38.113.5 netmask 0xffffffff broadcast 196.38.113.5 > inet 196.38.113.6 netmask 0xffffffff broadcast 196.38.113.6 > inet 196.38.113.7 netmask 0xffffffff broadcast 196.38.113.7 > inet 196.38.113.8 netmask 0xffffffff broadcast 196.38.113.8 > inet 196.38.113.9 netmask 0xffffffff broadcast 196.38.113.9 > inet 196.38.113.10 netmask 0xffffffff broadcast 196.38.113.10 > inet 196.38.113.11 netmask 0xffffffff broadcast 196.38.113.11 > inet 10.0.1.4 netmask 0xffffff00 broadcast 10.0.1.255 > inet 10.0.1.5 netmask 0xffffffff broadcast 10.0.1.5 > ether 08:00:08:00:08:00 > media: Ethernet autoselect (100baseTX <full-duplex>) > status: active > lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384 > inet6 ::1 prefixlen 128 > inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2 > inet 127.0.0.1 netmask 0xff000000 > > Note how 196.38.113.2 (the machine's real address) has a netmask of > 0xffffffe0, or 255.255.255.224, and the other addresses in this range (all > of them aliases) have 0xffffffff, 255.255.255.255. Then look at 10.0.1.4, > which is an alias too. It has a netmask of 0xffffff00, or 255.255.255.0. > Now look at 10.0.1.5, an alias used for serving intranet web content to > legacy machines. Again, a netmask of 0xffffffff. > > The reason for this is that 10.0.1.4, even though being an alias, is the > first address the machine handles on that subnet. Just as 196.38.113.2 is > the first address the machine has on the public subnet. > > Rule of thumb: First address on a subnet, alias or not, has the proper > subnet netmask. Every other address on the subnet following that, has > 0xffffffff, or 255.255.255.255. > > Will Thanks for clarifying this, Will. :) That was a very lucid and helpful explanation. - Mark To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-questions" in the body of the message
