A subnet mask of 255.255.240.0 means 20 bits must be assigned when you create the scope.

DHCP supports the use of Multicast scopes to support applications and clients that use multicast communications

You can create a scope in DHCP, and then define a user class for tablet devices. Assign the tablets to the user class with the IPconfig.exe tool. Finally, create a DHCP scope- level policy to assign a different lease duration for devices that match the condition of user class = laptop.

There are a number of possible solutions. One solution would be to manually assign IPv4 addresses to all clients in the branch. This would negate the need for DHCP, but it does make management of the organization’s IP address space more complicated. Us-ing DHCP, possible solutions include placing a DHCP server at the branch and configuring it with the required scope for that branch. This is probably the simplest solution, and requires no failover configuration.
If you preferred to allocate IPv4 addresses from the London offices, then using split scope would work well, with the Delay in DHCP Offer value set higher on the branch DHCP server so that it does not offer addresses unless the London DHCP server is unresponsive. You would need to take care to set this value correctly because obtaining an IP configuration over a wide area network link is slower than over a local area network link.

This scenario is probably best resolved by using DHCP failover. Configure a DHCP scope(s) for the London offices on one DHCP server, and then implement DHCP failover in load sharing mode, distributing the scope 50/50. This optimizes perfor- mance and helps to ensure high availability.