Unable to Ping / Shared Network

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Jul 7 19:24:41 UTC 2011

Barry Stear wrote:

A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

>Please reply directly to me because I seem to only be getting 
>digests of these emails (<mailto:bstear at gmail.com>bstear at gmail.com)

Ah, so not only have you not figured out how to make plain text 
emails (see the abominable double-address above), and not figured out 
that you don't need to quote the entire digest compilation (25k in 
this case !) in your reply, and not learned how to bottom post, and 
not learned how to change the subject line to something more 
informative than "dhcp-users Digest, Vol 33, Issue 8" ... you also 
haven't noticed the link at the bottom of EVERY email you get from 
the list :
Follow that link and you can change your options - for some reason 
it's probably set to digest mode.
Log into that page (have a password reminder sent if you don't know 
what it is), and on the membership configuration page you'll find 
"Set Digest Mode" as the second option down. Set it to off and click 
"Submit My Changes" at the bottom of the page.

Sorry, but in one message you've ticked nearly every box in the list 
of "bad netiquette that annoys me" !

>Simon.. I don't have OpenWRT on my Router but I do have DD-WRT. I 
>could not find where I can assign another IP address to my router, 
>at least not under Setup | Basic Settings.Perhaps if you could point 
>me in the right direction there.

I don't have one to hand, and I've only used DD-WRT once. On Open-WRT 
it's under LAN setup - you may have to go to advanced mode.

Hmm, the DD-WRT wiki is particularly unhelpful, but does have a link 
to this simulator :
 From that it appears there isn't a web interface for adding 
additional IPs to an interface. You will need to dive into the 
command line for it - it'll be a simple command, but no I don't know 
what it might be.
You'll have to ask over at the DD-WRT forum for that.

What you can do is setup a route under Setup -> Advanced Routing. The 
values you'll need are :
Subnet mask

>I figured this was a routing problem but I thought I wouldn't need a 
>route setup for the 100.X subnet since the 100.1 ip address is an 
>alias of Eth0:1 on Eth1 on my linux box. I just want to make sure 
>you understand that this is ONE interface card so I thought any 
>traffic from the 192.168.100.X subnet would just need to have 
> assigned as a gateway and my linux box would be doing 
>the routing internally.

OK, from the point of view of a device on the network, is a distant network - and vice versa. Ie, without 
assistance, a device configured in ONE of those networks cannot 
communicate with devices in the other.
So, you need routing in place to allow packets both ways.

Devices in the network are fine - for them the default 
router is a device that knows about the network. So they 
can send packets which will reach other devices both on that network 
and the wider internet.

But the other way round is not fine. Devices in the 
network are using the Linksys as their router. Since the Linksys 
doesn't know how to reach the network, packets 
addressed to that network will be lost - the Linksys will in fact try 
to route them out via the WAN and they will be dropped by the ISP if 
not before.
Similarly, for inbound packets which are replies to outbound 
connections from devices, whilst the Linksys will 
correctly apply the reverse NAT mapping - the packet cannot be 
delivered because it doesn't know how to route them.

>Maybe I am missing something the DHCP Handbook made it sound like 
>there was no routing configuration needed, and I believe 
>actually said to not setup routing.

Without seeing the statement in context it's hard to comment. However 
it could well have been talking about the DHCP aspects. For a DHCP 
**SERVER** attached to a shared network, it will work fine without 
any routing configured on the server or elsewhere - assuming the 
interface has the relevant additional addresses added to it.
Some messages are broadcast and so independent of the IP addresses on 
the interface. For unicast messages, the server has a suitable 
presence in each subnet and so routing is not required.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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