Netmasks: the truth is out there?

Steve Warwick ukla at
Tue Apr 29 10:03:04 PDT 2003

Hi All, 

I know someone out there knows the truth about netmasks. All the
documentation I can find on the net talks about "masking off" part of the IP
address and how you can tell network from subnet and host ids.

However, I have knowledgeable friend who is telling me that the netmask is
also dependant on how the hosting company has set up their routing tables
and incorrect netmasking on my part can reduce or halt my connection.

So the question is, who is right? Is it as simple as just masking off the
first three octets or does the netmask interact with the router?

Please point me to an authoritative source or if you have the time, a brief
description of if or how the netmask has to match the router, as I am not
sure what questions to ask my hosting company.


My current rc.conf entry looks like this (these ips are bogus) - no
broadcast and FreeBSD seems happy with this.

ifconfig_rl0="inet 123.456.789.111 netmask"
ifconfig_rl0_alias0="inet 123.456.789.112 netmask"
<+7 other aliases>

Thoughts, suggestions, links ?



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