[chbot] DHCP questions

[Paul Davey]

Hi Robin,

What does your network look like?

The key thing to know about VLANs on managed switches is that they are
the method used for separating your physical network into multiple
logical layer 2 networks, so two separate VLANs behave mostly as if
they are separate switches, and unless you have routing rules set up
things cannot communicate between them.


On Wed, Aug 17, 2022 at 2:33 PM Robin Gilks <gb7ipd at gmail.com> wrote:
>
> Hi Volker
>
> I'm up to speed on basic DHCP, I've been running my own server for over 20 years including PXE booting of dumb clients, fixed MAC address mapping, blacklist and white list configurations.
>
> What I don't get is how to prevent a client that roams from one AP to another from losing its DHCP allocated address due to either not refreshing or starting from scratch on each roam.
>
> I feel it's something to do with VLANs and managed switches but I just don't know enough of the vocabulary to hold a conversation
>
>
> On Tue, Aug 16, 2022 at 5:28 PM Volker Kuhlmann <list57 at top.geek.nz> wrote:
>>
>> 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.
>>
>> HTH
>>
>> Volker
>>
>> --
>> Volker Kuhlmann
>> http://volker.top.geek.nz/      Please do not CC list postings to me.
>>
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