Netmasks: the truth is out there?

Alex Trull atrull at
Tue Apr 29 10:26:59 PDT 2003

* Steve Warwick <ukla at> [2003-04-29 10:03:02 -0700]:

> 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.
> defaultrouter="123.456.789.1"
> ifconfig_rl0="inet 123.456.789.111 netmask"
> ifconfig_rl0_alias0="inet 123.456.789.112 netmask"
> <+7 other aliases>
> Thoughts, suggestions, links ?

Well, I can't help you with the theory.

But I've seen a mistake in the rc.conf cutting you spamme:

ifconfig_rl0_alias0="inet 123.456.789.112 netmask"
should read:
ifconfig_rl0_alias0="inet 123.456.789.112 netmask"

as should any other secondary aliases to subordinate a prime address 
(where they would in reality share the same netmask)

to nail the concept home, if you had another set of aliases for a different network:

ifconfig_rl0_alias15="inet 321.456.987.11 netmask"
ifconfig_rl0_alias16="inet 321.456.987.12 netmask"
ifconfig_rl0_alias17="inet 321.456.987.13 netmask" 

I hope that helps, and you find answers to your other questions. 

Alex Trull

Systems and Network Administrator & Information Commissioner
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