Configuring IP address aliasing

[Mike]

Greetings,

I have 2 Redhat-9 servers that I'd like to migrate to FreeBSD in the 
next 3 months.  So I've set-up a test server (FreeBSD-4.9 STABLE) and 
I'm in the midst of loading 3rd party applications (via ports) to test 
their operation compared to the RH-9 servers.

I have a question about the configuration of IP aliases, so here is some 
background information.

Here is the real interface (192.168.1.40):

# ifconfig -a
ed0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         inet 192.168.1.40 netmask 0xffffff00 broadcast 192.168.1.255
         ether 00:80:ad:91:9a:bd
lp0: flags=8810<POINTOPOINT,SIMPLEX,MULTICAST> mtu 1500
ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 552
faith0: flags=8002<BROADCAST,MULTICAST> mtu 1500
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
         inet 127.0.0.1 netmask 0xff000000

Now I try to add the alias (192.168.1.41):

# ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.0 broadcast 
192.168.1.255
ifconfig: ioctl (SIOCAIFADDR): File exists
[using same netmask as real interface]

However, THIS WORKED:
# ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.255 broadcast 
192.168.1.255
[using a netmask of 255.255.255.255]

And here is the ifconfig that shows the alias in place:

# ifconfig -a
ed0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
         inet 192.168.1.40 netmask 0xffffff00 broadcast 192.168.1.255
         inet 192.168.1.41 netmask 0xffffffff broadcast 192.168.1.255
         ether 00:80:ad:91:9a:bd
lp0: flags=8810<POINTOPOINT,SIMPLEX,MULTICAST> mtu 1500
ppp0: flags=8010<POINTOPOINT,MULTICAST> mtu 1500
sl0: flags=c010<POINTOPOINT,LINK2,MULTICAST> mtu 552
faith0: flags=8002<BROADCAST,MULTICAST> mtu 1500
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
         inet 127.0.0.1 netmask 0xff000000


QUESTION: Why do I use a different netmask (255.255.255.255) for a IP 
alias on FreeBSD?  Why isn't 255.255.255.0 used?

When I set-up IP aliases using Linux, I use the same netmask 
(255.255.255.0) as the real IP.

The IP aliasing of my BSD server works but WHY is it working?

I found the reference to using the 255.255.255.255 netmask via Google, 
however, I have Michael Lucas's book "Absolute BSD" and reference on 
pages 103 & 104 (on IP aliasing) clearly show using the same netmask as 
the real interface when creating IP aliases.

What am I missing?

Thanks,
Michael