DHCP or not ?

Davek0974 · Jun 12, 2017

I'm just getting ready to revamp our IT setup, 20 users, various printers, scanners etc, IP phones, 2 servers.

At present its a mess of static IP and DHCP (handled by our router)

Should i look to swing everything apart from the servers and router to DHCP or bias towards statics?

Is there a down side to DHCP?

Should I keep a small pool of static addresses at the bottom for servers and printers or are printers ok as dynamic too?

Thanks

Kixo · Jun 12, 2017

Small number of static IPs for things like routers and servers, then DHCP for everything else. If you have a lot of devices set the lease to something like 1 day or 3 days. Only things that should be static are large common devices likes printers servers routers etc. DHCP doesn't have any drawback I can think of!

Davek0974 · Jun 12, 2017

Thanks

Just been ploughing the 'net and DHCP reservations came up a fair bit - use or just go full dynamic??

Kixo · Jun 12, 2017

It depends on your OCD, being an engineer I like to know all my statics are between .1 and say .50 and anything above .51 I know it's a non critical infrastructure device. But if it's a small network and you don't need to know to much, then yes reservations are good, just you can't choose the IP it reserves... That's an OCD thing not a actual problem

Kixo · Jun 12, 2017

Also DHCP is good as the is less for you to do.... DNS names should be the thing you use more, as the network grows DHCP grows with it, static grows in problems

Davek0974 · Jun 12, 2017

Ok, thanks,

looking at our router (Draytek Vigor), it has a table for reservations - seems you can fill in an IP from the pool and assign it to a MAC address.

I'll carry on with my documenting and see how we go. There are a few devices out in the IP field that I'm unsure of how to alter the IP or switch to dynamic, will have to research those, mostly older multi-port print-servers etc.

Kixo · Jun 12, 2017

That's the problem with static you have to know what is in use and what the IP is, DHCP you just fire and forget, drayteks are good!

Davek0974 · Jun 12, 2017

If we cannot change a couple of items to dynamic - can we just reserve the static IP they had in DHCP reservations or will they no longer connect?

KM-Tiger · Jun 12, 2017

Davek0974 said:
If we cannot change a couple of items to dynamic - can we just reserve the static IP they had in DHCP reservations or will they no longer connect?

If the DHCP server can do reservations then yes you can. What that means is that the device (identified by MAC address or hostname) will always get the same IP address. And has the advantage that you can set DHCP options, eg default gw, DNS servers, time server, using DHCP.

soundengineeruk · Jun 12, 2017

The logic I use is that anything that is fixed like servers, printers, desktop will be assigned a reservation. Everything else is dynamic (laptops, mobile etc)

DHCP real issue is not big enough pool of IP, especially with the number of devices per user.

Davek0974 · Jun 13, 2017

soundengineeruk said:
DHCP real issue is not big enough pool of IP, especially with the number of devices per user.

Yes, thats where i am at now - we have mostly statics but a small pool of DHCP - very hard to expand as moving fixed IP printers etc is a PITA and someone stuck a server up high in the numbers too

soundengineeruk · Jun 13, 2017

Davek0974 said:
Yes, thats where i am at now - we have mostly statics but a small pool of DHCP - very hard to expand as moving fixed IP printers etc is a PITA and someone stuck a server up high in the numbers too

I'm assuming you have only 254 available ip addresses? (192.0.0.0/24).

If yes, it may be worth increasing your IP address subnet from /24 to 21 giving you range from 192.168.0.1 to 192.168.7.255 (2.046 IP addresses)

Then separate the network
192.168.0.1 to 254 - core (servers, printers, etc)
192.168,1.1 to 254 - lan network
192.168.2.1 to 254 - wifi network.

Then you still have room to expand after that. In fact you can do 192.168.0.0/16 which would give you 65,534 IP address from 192.168.0.1 to 192.168.255.254.

Unfortunately nothing can be done about PITA bit

Davek0974 · Jun 13, 2017

Yes i was thinking that last night but how are the ranges bridged so they appear as one network?

soundengineeruk · Jun 13, 2017

Davek0974 said:
Yes i was thinking that last night but how are the ranges bridged so they appear as one network?

It be broadcasted from single interface from DHCP server. If you have multiple interfaces on your DHCP server and multiple switches, then configure 1 range per interface to each switch

Interface 1 - 192.168.0.0/24 - core switch
Interface 2 - 192.168.1.0/24 - lan network
Interface 3 - 192.168.2.0/24 - wifi network.

If single interface with single switch can still broadcast the range, but DHCP could be little more tricky to set up

KM-Tiger · Jun 13, 2017

Davek0974 said:
but how are the ranges bridged so they appear as one network?

They are one network because of the /21 suffix, which is equivalent to a netmask of 255.255.248.0

Code:

ipcalc 192.168.0.0/21
Address:   192.168.0.0          11000000.10101000.00000 000.00000000
Netmask:   255.255.248.0 = 21   11111111.11111111.11111 000.00000000
Wildcard:  0.0.7.255            00000000.00000000.00000 111.11111111
=>
Network:   192.168.0.0/21       11000000.10101000.00000 000.00000000
HostMin:   192.168.0.1          11000000.10101000.00000 000.00000001
HostMax:   192.168.7.254        11000000.10101000.00000 111.11111110
Broadcast: 192.168.7.255        11000000.10101000.00000 111.11111111
Hosts/Net: 2046                  Class C, Private Internet

Note the network and broadcast addresses. Everything in between is on the same network.

Davek0974 · Jun 13, 2017

Hmm, bordering on my knowledge scope now (embarrassed) - never strayed away from the classic 255.255.255.000

So my net runs from 192.168.000.001 to 192.168.000.255, so the last three mask digits control the visible addresses ...001 to ...255 and being ...000 allows me to see all of that range?

If i change my netmask to 255.255.248.000 I can then use ranges 192.168.000.001 to 192.168.000.255 all the way up to 192.168.007.254 and all in between like 192.168.003.234 and 192.168.005.123 ??

Do i grasp that ok?
(should have studied this stuff

)

KM-Tiger · Jun 13, 2017

Davek0974 said:
Do i grasp that ok?

Yes.

It is confusing unless you do this stuff all the time.

Google 'CIDR notation' if you want to know more.

soundengineeruk · Jun 13, 2017

Davek0974 said:
Do i grasp that ok?
(should have studied this stuff )

Yeah'ish. Plan before making any changes to avoid losing yourself.

Davek0974 · Jun 14, 2017

Of course.

Is it a 1 for 1 change in the subnet mask :-

Mask 255.255.255.0 allows 192.168.000.000 to 192.168.000.254

Mask 255.255.254.0 allows 192.168.000.000 to 192.168.001.254

Thanks

KM-Tiger · Jun 14, 2017

Correct.

Code:

ipcalc 192.168.0.0 255.255.254.0
Address:   192.168.0.0          11000000.10101000.0000000 0.00000000
Netmask:   255.255.254.0 = 23   11111111.11111111.1111111 0.00000000
Wildcard:  0.0.1.255            00000000.00000000.0000000 1.11111111
=>
Network:   192.168.0.0/23       11000000.10101000.0000000 0.00000000
HostMin:   192.168.0.1          11000000.10101000.0000000 0.00000001
HostMax:   192.168.1.254        11000000.10101000.0000000 1.11111110
Broadcast: 192.168.1.255        11000000.10101000.0000000 1.11111111
Hosts/Net: 510                   Class C, Private Internet

'ipcalc' is a LInux command line tool. There is an online version:

http://jodies.de/ipcalc

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DHCP or not ?