NAT router confusion

James Bowman Sineath, III sineathj1 at
Fri Jun 24 10:49:44 GMT 2005

My understanding is that the netmask ( as you put it) is only 
to determine how much of the IP address is used for the subnet address. I'm 
a newb with this as well, so please, someone correct me if i'm wrong. If 
your IP is and your netmask is, then only the 
last 8 bits of your IP (the last .10) is usable for a specific host on the 
network and the first 24 bits are used for the network address and subnet 
address. In binary:


would be your netmask and only the trailing 0's can be used for a host 
address. This could also be expressed as using CIDR. Let me 
try to give you another example:

if your IP range was to and netmask was then, in binary, the netmask would look like this:

Being that you are using as the network address, the first 
three 1's in the last 8 bits of the netmask would be your subnet addresses. 
So you could use., *.64, *.96, *.128, *.160 and *.192 for 
subnet addresses and the IPs between all of those (except the last IP, so 
you can only assign 30 per subnet since the last IP is used for broadcast) 
can be assigned to hosts.

Hopefully that (correctly) clears up any confusion involving subnets and 
netmasks. Like I said, I'm new at as well, so please correct me if I am 

----- Original Message ----- 
From: "Ulf Magnusson" <ulfma629 at>
To: <freebsd-questions at>
Sent: Friday, June 24, 2005 6:25 AM
Subject: Re: NAT router confusion

> ----- Original Message -----
> From: "Michael H. Semcheski" <lists at>
> Date: Friday, June 24, 2005 1:46 am
> Subject: Re: NAT router confusion
>> On Thursday 23 June 2005 07:43 pm, Ulf Magnusson wrote:
>> > Is this router really some switch/router hybrid? Or..? Bleh, someone
>> > please sort this out for me. I realize this isn't strictly
>> > FreeBSD-related, but I simply couldn't think of a better place to
>> pick> brains, so I hope I'll be excused :)
>> It is a switch / router hybrid.  If the traffic is going to an
>> address on the
>> same network, its a switch.  If the traffic is going to an address
>> on a
>> different network, its a router.
>> If you understand that concept, then you should have a pretty good
>> idea of how
>> the system works.
>> I do not have a complete enough understanding of IP networks to
>> explain this
>> in specific detail.  I think the key is that the computer
>> generating the
>> traffic looks at the netmask for the sending interface (eg,
>> and uses this to determine if the endpoint of the traffic is on the
>> same
>> network or not.  If it is, it sends the traffic directly to the
>> host.  If it
>> is on a different network, it forwards the traffic to the gateway
>> address.
>> Mike
> Thanks, I think I understand how it works now. I guess it's basically
> like an ordinary router that pretends it's a switch for all addresses
> that appear on the same local network. It looks at the destination
> address in IP packets and the address of the sending system and goes
> into switch mode if they both appear on the same subnet (which is pretty
> much verbatim what you said, when I think about it).
> I'll throw another short question in the mix while I'm at it.. perhaps I
> should rename the thread "Switching/routing questions from a curious
> networking newbie" :-)
> Do switches gain anything by having full-duplex connections to hubs? I
> understand there must be a performance benefit when you connect a host
> directly to a switch, but won't the half-duplex connections of the hosts
> to the hub become a bottleneck?
> Ulf
> _______________________________________________
> freebsd-questions at mailing list
> To unsubscribe, send any mail to 
> "freebsd-questions-unsubscribe at" 

More information about the freebsd-questions mailing list