Re: IP Address problems

From: John Santos (JOHN_at_egh.com)
Date: 01/17/04 

Date: Fri, 16 Jan 2004 20:28:57 -0500

Whoops. Accidently mailed this to DLP instead of posting it. Sorry.
("You" below generally applies to the original poster, Chris, rather
than DLP...)

On 16 Jan 2004, DL Phillips wrote:

> chrisgaze@msn.com (Chris Gaze) wrote in message news:<57512a77.0401160102.4afe92d4@posting.google.com>...
> > AH 10.100.100.100 172.16.26.126
> > AH 127.0.0.1 127.0.0.1
> > AN 172.16.26.0/25 172.16.26.65
>
> It looks to me like 172.16.26.126 is not in your 172.16.26.0/25 network,
>
> > WEA0 172.16.26.65 255.255.255.128 2412292 1915754 1500
>
> Because, as your network mask shows,
> ~.128/25 is like ~.0/24
> ~.129/25 is like ~.1/24
>
> So, change your network to /24 or move your router and gateway up into your network.
>
> DLP

Uh, no... These numbers are decimal, so 172.16.26.0/25 ranges from
172.16.26.0 to 172.16.26.127, so includes .126.

(172.16.26.128/25 would range from .128 to .255...)

I don't really understand subnetting rules, and maybe some of this
is obsolete, but I was given to understand that if you subnet a
class C network, the first and last subnets are unavailable, and
you can only use the middle ones. I never heard an explanation of
*why* this is true that made any sense to me, though.

Assuming this is true, there are many ways to slice and dice a
class C network, but /25 isn't one of them.

Here's a little table:

mask subnets valid hosts per total hosts
size subnets subnet
/24 1 1 254 254 (Normal, non-subnetted class C)
/25 2 0 126 0
/26 4 2 62 124
/27 8 6 30 180
/28 16 14 14 196
/29 32 30 6 180
/30 64 62 2 124
/31 128 126 0 0

(The 1st and last host number in each subnet, e.g. .0 and .255
for a /24 network, or .0, .127, .128 and .255 for a /25 network,
are reserved for the network address and broadcast address and
so can't be used for hosts, so there are 2 fewer hosts per subnet
than you would expect.)

Maybe your /25 should really be a /26? Then the host and the
router would be the 1st and last valid host addresses in the
172.16.26.64/26 subnet. 

-- 
John Santos
Evans Griffiths & Hart, Inc.
781-861-0670 ext 539

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