Netmasks: the truth is out there?
iaccounts at northnetworks.ca
Tue Apr 29 10:51:27 PDT 2003
> 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.
The netmask defines logical groups of IP addresses within a given range.
It allows an ISP to subdivide up IP blocks into 'subnets' that are
classified as different networks.
The subnet doesn't really 'interact' with the router, but the router will
know where to send a packet based on the IP and netmask. To a router, the
netmask is as important as the IP address. Configuring improper netmasks
into a routers routing table can be catastrophic (depending on the purpose
of the router).
>From your end, unless you have a static IP from your ISP, you probably
need not be concerned with netmasking because DHCP will take care of this
for you. Otherwise, with a static IP, you will have to enter in this
manually. INcorect configuration of the netmask will likely cause your
Internet connection to fail.
In order to learn about subnetting, search google for 'subnetting', or try
to find some online Cisco documentation. It was such Cisco docs and books
that enabled me to literally 'subnet in my head'.
> 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 255.255.240.0"
> ifconfig_rl0_alias0="inet 123.456.789.112 netmask 255.255.255.0"
> <+7 other aliases>
> Thoughts, suggestions, links ?
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