Cisco Router DHCP host range

[imranbhatti22006]

Dear Members,

I've a unique case for one of my customer to discuss, He wants to configure DHCP on Cisco router as dhcp server.

his address range are crazy. But he doesn't want to touch / change network right now cause production is live and he can't afford downtime. here are details:

He wants to handover following host range to clients on network, Tell me Excluded range please, I couldn't calculate.

Host Range,
10.255.250.1 /8

10.255.253.254 /8

Router dhcp output below.

DHCPRouter#show ip dhcp pool HOLAN

Pool HOLAN :
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 16777214
Leased addresses : 3
Pending event : none
1 subnet is currently in the pool :
Current index IP address range Leased addresses
10.0.0.2 10.0.0.1 - 10.255.255.254 3
DHCPRouter#

[MHM Cisco World]

If i understand you, 

Check dhcp smart relay feature it will help you in downtime

[David Ruess]

Hello,

If you only needs hosts in the range you specified up top then you could create a DHCP pool like the one below:

10.255.250.0 255.255.248.0

However, the range you specified also falls in between 2 subnets so you would need to adjust.

IP Address:	10.255.248.0
Network Address:	10.255.248.0
Usable Host IP Range:	10.255.248.1 - 10.255.251.254

You can exclude any addresses you want by using the DHCP excluded address command.

ip dhcp excluded-address <start address> [<end address>]

If you want the DHCP pool to hand out addresses the hosts need to be on the same LAN or VLAN of the subnet configured to participate and get an address as well as reachability from where they are at. If not then you need to configure a helper address on the router port pointing to the LAN segment the hosts are connected to with the DHCP server address. 

Hope that helps

-David

[Georg Pauwen]

Hello,

I think @David Ruess is right on the money. According to my calculation, the excluded range(s) and the DHCP pool would look like this:

ip dhcp excluded-address 10.250.248.1 10.255.248.254
ip dhcp excluded-address 10.250.249.1 10.255.249.254
ip dhcp excluded-address 10.250.254.1 10.255.254.254
ip dhcp excluded-address 10.250.255.1 10.255.255.254
!
ip dhcp pool NET
network 10.250.248.0 255.255.248.0
default-router 10.250.250.1

[paul driver]

Hello 
ip dhcp excluded-address 10.250.248.1 10.255.248.254
ip dhcp excluded-address 10.250.249.1 10.255.249.254
ip dhcp excluded-address 10.250.254.1 10.255.254.254
ip dhcp excluded-address 10.250.255.1 10.255.255.254

Why are you not excluding .0 and 255 ? 

[imranbhatti22006]

Hi Mr. Driver,

.0 and 255 are network / broadcast addresses, if i'm correct.

[paul driver]

Hello
.0 &255 can be subnet-broadcast addressing when you have/24 address range
However anything larger like a /8 then you would still have .0 & .255 (subnet/broadcast) but ONLY just the one instance of them anything inbetween will be valid ip addresses including any other .0 & .255.

[imranbhatti22006]

Thank you so much folks,

for your valuable input. And i'll discuss given solution with the IT Manager and I'm sure we don't have any other choice to opt.

will keep you posted if we face any other challenges along the way.

[Leo Laohoo]

H0ly shlt!  A /8 network!

Is this a flat network?  

Good luck when someone drops a worm into the network.  It is going to be a kiss-your-a**-goodbye moment.  

[imranbhatti22006]

LOL... that's so true.

[paul driver]

Hello
By default dhcp will exclude its own ip address that’s being used for default-gateway so you no need really need to exclude that, also the exclusions depend on what address are currently being used within that very large subnet, (network devices, rtrs, switches/fw/servers/aps, printers etc) if they all reside in that exclsuion then it will be:

10.255.248.0 - 10.255.255.254

[imranbhatti22006]

That's so spot on arguements guys, anyways we've submitted them new Proposal with revised IP Schema and restrucuring of their entire network. Let's see what happens. Thought, this issue has been sorted out.

1- Their total LAN host range is max 1022 hosts.

2- They've Remote branches, connected via mpls, branch router will handle them. not more than 150 hosts / branch.

3- Plus, they've some equipment dedicated for vsat live tv. Max 50 IP's for them. so its all in good order now.

Thanks again everyone