Limitations Creating DHCP Reservations

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The DHCP server must service the subnet where the reservation is being created.

These three rules guide you in creating reservations:

1. You may create a reservation in any scope range.
   
   This can include Excluded ranges.
2. You may create a reservation in a subnet range even if the reservation falls outside the actual scope Start and End addresses for that subnet.
3. You cannot create reservations outside the subnet range of any of your existing scopes.

As a simple example, take a DHCP server that has no existing scopes:

• Let's say you have a standard Class C subnet for 192.168.1.0 with mask 255.255.255.0.
• You can create a scope for the first half of the subnet so that the scope has a starting address of 192.168.1.1 and an ending address of 192.168.1.150.
• You can now create a reservation for the address 192.168.1.25. (Rule 1)
• You can also create a reservation that lies outside the scope but still lies in the subnet range.
  
  For example, reserve 192.168.1.200. (Rule 2)
• You cannot create a reservation for the address 192.168.2.4, which lies outside the subnet of any existing scope. (Rule 3)