From: paul.cocker@bt.com
Date: Tue Jul 03 2007 - 07:17:17 ART
Hi Gary,
So basically, the whole point of the netmask statement is belt and
braces approach to stop you advertising the broadcast and network
address? Why would this be necessary?
The reason I came across this, was noticing how it's used on a network I
work in.
There are several pools for different things, and they're advertised out
in BGP.
The pools are like this, almost always /30's
Ip nat pool XYW x.y.z.5 x.y.z.8 netmask 255.255.255.252
The thing is, when we do this, we only get 2 usable addresses from it.
This seems very wasteful. Given the company in question owns at least
x.y.z.a /24 (in fact, I don't see that it would matter even if they
didn't own the whole block).
It would seem better to use...
Ip nat pool XYW x.y.z.5 x.y.z.8 netmask 255.255.255.0
Then something along these lines within BGP
Network x.y.z.5 mask 255.255.255.252
As in this case, the broadcast and network address are superfluous,
aren't they?
Any comments?
Regards,
Paul
-----Original Message-----
From: Gary Duncanson [mailto:gary.duncanson@googlemail.com]
Sent: 03 July 2007 10:56
To: Cocker,P,Paul,JGAN34 R
Cc: ccielab@groupstudy.com
Subject: Re: ip nat pool command
It won't. You might be confusing subnet information contained in routing
updates and IP header info.
The netmask helps define the range of IP addresses assigned by your ISP
for
dynamic NAT. The hosts internally that match on the ip nat inside source
list ACL will use the ip addresses defined in the ip nat pool command
for
translation.
Do show ip nat statistics to see the start and end IP address of your
NAT
pool in the output.
----- Original Message -----
From: <paul.cocker@bt.com>
To: <dotcomvijay@gmail.com>; <ccielab@groupstudy.com>
Sent: Tuesday, July 03, 2007 10:27 AM
Subject: RE: ip nat pool command
> Hi Vivjay,
>
> Thanks for you response.
>
> I thought there was no field in the IP header for subnet mask, so I'm
a
> little confused how it would enter the ISP with a particular subnet
> mask?
>
> Regards,
> Paul
>
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Vijay babu
> Sent: 03 July 2007 09:51
> To: ccielab@groupstudy.com
> Subject: Re: ip nat pool command
>
> My understanding is...
>
> When the nat pool is defined with
> Ip nat pool XYW 1.1.1.1 1.1.1.50 netmask 255.255.255.0
>
> The nated packet leaving your network is give an ip 1.1.1.1 with a
> subnet
> mask
> of 255.255.255.0. This also means the ISP has provided you a 1.1.1.0
> (entire
> class c)subnet
> for your Internet access.
>
> If nat pool is defined with
> Ip nat pool XYW 1.1.1.1 1.1.1.50 netmask 255.255.255.128
>
> The nated packet is give an ip 1.1.1.1 with network mask
255.255.255.128
> This means your ISP has given you 1.1.1.1 till 1.1.1.126 range of
> ip address for your use.
>
> Cheers
>
> Vijay
>
>
>
>
> On 7/3/07, paul.cocker@bt.com <paul.cocker@bt.com> wrote:
>>
>> Hi All,
>>
>> I'm a little confused what exactly the netmask command does
>>
>> Ip nat pool XYW 1.1.1.1 1.1.1.50 netmask 255.255.255.0
>>
>> How does that differ from
>>
>> Ip nat pool XYW 1.1.1.1 1.1.1.50 netmask 255.255.255.128 ?
>>
>> What's the purpose of it?
>>
>> Regards,
>> Paul
>>
>>
>
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