"Since modern DHCP servers ping check an ip before they assign it," A quick icmp check before allocation seems like a logical idea until you really think about

"Since modern DHCP servers ping check an ip before they assign it," A quick icmp check before allocation seems like a logical idea until you really think about it. A DHCP server on the same subnet as the allocation could try ARP first and then resort to ICMP. If the request is relayed then it can't use ARP and can only rely on something like ping. However, all that takes time. DHCP was invented in a time when a local switched LAN should have a maximum RTT of around 20 to 100ms across itself. Nowadays I expect a RTT to Google public DNS of around 30ms from my laptop over wifi. How long are you prepared to wait for the test before allocation? That test takes time. Do you send one ping and wait for a second? That's an additional second on roaming and when you open the lid on your laptop and so on. Do you actually wait three seconds - to be really sure, which is even worse. You also have the overhead of DHCP itself and the various reconfiguration systems doing their stuff. Windows DHCP, Novell's (back in the day), Kea, dhcpd and the rest can all do an ICMP check but I don't recommend it. If you are doing DHCP and you own your network then use DHCP snooping on your switches to keep it regular.