Excluding adresses from DHCP pool, why?

[spree]

Hello!

 

I am setting up a topology which looks like this: 

 

 

R1 default gateway is 192.168.10.1. 255.255.255.0

From my understanding I need to exclude the following adresses from the pool of DHCP:

192.168.10.1 (the default gateway)

And the first adress, 192.168.10.0 

And the last, 192.168.10.255

 

Why is it important to exclude these adresses? If the client (PC1) were to be assigned 192.168.10.0 (network-ID) what would happen?

Or the 192.168.10.255 broadcast?

 

Thanks!

[SupaKomputa]

Yes you got to exclude the gateway address from dhcp pool.

Since the gateway doesn't use dhcp for it's ip, you should start from .2 and so on.

[brwainer]

The DHCP server in Cisco Routers doesn’t do any checks for whether an IP is in use, it only uses its own leases list. It doesn’t even check if the router’s IP is in the range.

 

Many alternative DHCP servers do an ARP and/or Ping check on IPs before it offers one. And many DHCP clients (including all iOS devices) will do the same before accepting an IP offer. But you shouldn’t rely on this, and in Cisco Routers you should always put in an exclude for any static IPs in use, including the router itself.

 

I don’t believe you need to put in an exclude for the network and broadcast IPs, I’ve never seen it in production.

[eece_ret]

255 is broadcast address. https://en.wikipedia.org/wiki/Broadcast_address

0 is network identifier.  If you assign to a client the client OS local network stack will not know what subnet it is on and will not know how to forward, L2 (ARP -> Ehternet) or L3 (Forward to gateway for Forwarding).  

.1 in your case, in use by gateway.  Client receiving this address would bork all L3 flows in teh L2 segment as you would have an IP conflict with gateway.

 

To use a .0 you could rock 192.168.10.0/23 which gives you a useable range, 192.168.10.1-192.168.11.254.  192.168.10.0 is right in teh middle and is acceptable.

 

Hope that helps.