[chbot] DHCP questions

Volker Kuhlmann list57 at top.geek.nz
Tue Aug 16 06:27:17 BST 2022

Hi Robin,

> Didn't have time last night to follow all the information about DHCP and
> network spanning etc. The conversation was getting outside my current
> knowledge base so if someone can point me at a couple of relevant articles,
> so I can digest the basics and can then have a meaningful chat with our
> networking suppliers on how their product works, that would be great.

Generic beginner rundowns on DHCP should be on the net, but I don't know
any off-hand. Ignore DHCP6 (DHCP for IPv6).

The basics are simple enough though. DHCP is a protocol which endpoints
can use to obtain an IP address. It uses broadcasts, for obvious
reasons. The DHCP server grants a lease of an IP address which is valid
for a certain time, after which the client must request a new one.
Servers can be configured to allocate a static IP based on the MAC
address of the client.

Things like your ATA basically always default to DHCP - least messing
around for the manufacturer. If they default to a fixed IP it's probably
printed on the label. The box's web interface may allow to configure
anything, factory reset should undo all that.

It's easiest to start with DHCP. You can check with nmap -sP .../24
where the thing might be address-wise.

There's a problem when the box's IP address is not on your computer's
network (with DHCP that doesn't happen). In that case alias another IP
address to your network interface (ip command, ifconf has been
deprecated for many years and may not be installed by default). Then run
nmap. You can alias a /16 if you need to. Pick any number for your own
interface but not the one the box has. 33..219 should be OK. Cake if
you collide...

This holds for any network box, but things get a tad more complicated if
there is more than one RJ45 shaped hole in it.



Volker Kuhlmann
http://volker.top.geek.nz/	Please do not CC list postings to me.

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