DHCP Not Working

[Conor Hughes]

When I try to create a DHCP pool with a specific network it will automatically change the final part of my IP address to a 0

e.g. network 193.13.32.8 255.255.255.224

will become:

network 193.13.32.0 255.255.255.0

Above is a copy of my network. i have looked around and have no idea what the issue could be

[M02@rt37]

Hello @Conor Hughes 

With mask 255.255.255.224, /27 mask!, every "32" you have a netowrk address ; meaning :

1/ 193.13.32.0 255.255.255.224

2/ 193.13.32.32 255.255.255.224

etc...

so .8 is in the subnet 193.13.32.0/27

[Conor Hughes]

Hi M02@rt37

so am i not able to use DHCP pools to distribute IP address on a Variable Length Subnet?

[M02@rt37]

@Conor Hughes 

just use the correct mask ...

Start point : How many host I need on my subnet ?

[Conor Hughes]

so on Subnet D i need 5 hosts, so i used a /29 mask because that gives me 6 usable hosts

on Subnet E i need 19 hosts so i used a /27 mask, that gives me 30 usable hosts

and finally for Subnet F i need 21 hosts so i used a /27 mask again.

and as for the last three subnets they don't have any hosts so they are all /30

so the total usable hosts allocated for the network is 72.

am i just missing something super obvious?

[Richard Burts]

The OP says

e.g. network 193.13.32.8 255.255.255.224

will become:

network 193.13.32.0 255.255.255.0

I am confused why they think this is the case. Clearly the show run indicates that the network is network 193.13.32.0 255.255.255.224.

The OP seems to think that DHCP has a problem with Variable Length Subnet. That is not true. DHCP works quite well with  Variable Length Subnet. In doing this you need to be careful about specifying the network. It seems to me that the issue for the OP is that specifying 193.13.32.8 turns into 193.13.32.0. If they really want the network to be 193.13.32.8 then they can not use mask 255.255.224. Using that mask means that 193.13.32.8 is a host address and not the network address. Try the configuration using mask 255.255.255.248 and see what happens.

[M02@rt37]

Exactly @Richard Burts !!!

IP Plan should be designed first, regarding the need of hosts...

[Richard Burts]

Yes we agree. Having a plan that clearly identifies which addresses are network addresses and which addresses are host addresses would be very helpful. Once this plan exists it becomes easier to do the correct configuration.

The OP seems to believe that the address specified as network address being changed was a problem. We have clarified that it is important to identify which addresses are network addresses and which addresses are host addresses. And clearly if the configuration statement specifies a host address, it will be changed to the appropriate network address.

The OP has posted additional details about the networks and the addressing for those networks. That should work fine. The OP asks "am i just missing something super obvious?". I am not clear if there is still confusion about configuring DHCP for variable length subnets. 

[Conor Hughes]

Hi Richard

I think i must be misunderstanding something. i had assumed that when i type:

Network 193.13.32.8

I'm telling DHCP what the network address is and the subnet mask will indicate the amount of hosts that need to be distributed.

if i where to type :

Network 193.13.32.8 255.255.255.248

am i not telling DHCP to only distribute 6 usable hosts?

that being said, i tried what you said

the network address stayed this time. but unfortunately none of my host have had a IP address distributed to them

[M02@rt37]

@Conor Hughes 

Do you have an IP plan ? Do you have the number of hosts on each segment ? 

That's the start point. You will adjust MASK after that...

[Conor Hughes]

This was the first thing i created, before my topology or implementation

[Richard Burts · Accepted solution]

Thanks for the additional information. I am glad that you created this plan. Unfortunately the logic of the plan is flawed. In particular the plan for subnet E is not correct. The first address for that address/mask is not .9 but would be 193.13.32.1 and clearly overlaps with the other subnets.

[Conor Hughes]

I see that i didn't design my network from most hosts to least

i have configured it correctly now

much apricated 

[Richard Burts]

You are quite welcome. Thank you for marking the discussion as solved. This will help other members of the community to identify discussions that are helpful. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[Richard Burts]

In my previous responses I had missed this post. And I believe that the point of this post is important enough to deserve its own response. You say "I'm telling DHCP what the network address is". That is pretty much true, but you must be sure that the address you specify is really a network address and not a host address (which it is in your post). To do this you need to consider the mask along with the address. The mask will tell how many binary 0 and how many binary 1 are in the address. If the address has contiguous 1 (all 1s) to the left and contiguous 0 (all 0) to the right then it is a network address. If there is a mixture of 1 and 0 then it is not a network address.

Your plan was quite logical and was based on the assumption that subnets could start and end anywhere, as long as they did not overlap some other subnet. But that is not the case. The subnet must begin on a binary boundary (all 1s to the left and all 0 to the right).