What hypervisor are you using? If you're running a lab it would be best to keep the VMs in a host only virtual network, these also normally have their own DHCP so you you will need to turn that off in the hypervisor settings before deploying the SBS DHCP.
Is this for a cert? Only asking as MS are killing off SBS.
In addition to what tr2012 said: Your clients get there IP from the router, so there is no use for a second DHCP server in your network. If want to set it up anyway, it needs a different IP range to prevent double addresses. The SBS (as every server) should have a dedicated IP anyway and not getting it from the router.
What they said but I believe it actually needs to be a different subnet if I'm not mistaken.. and yeah Microsoft is killing off all SBS and home server editions for what they call an easier pricing model..