Configure Network Services - MCSA Windows Server 2012 R2: Installation and Configuration Study Guide - page 119

Exclusions are useful for addresses that you don't want to participate in
DHCP at all. Reservations are helpful for situations in which you want a cli-
ent to get the same settings each time they obtain an address.
An address cannot be simultaneously reserved and excluded. Be aware of
this fact for the exam, possibly relating to a troubleshooting question.
using reservations and exclusions
Deciding when to assign a reservation or exclusion can sometimes be confusing. In prac-
tice, you'll find that certain computers in the network greatly benefit by having static IP
network information. Servers such as DNS servers, the DHCP server itself, SMTP serv-
ers, and other low-level infrastructure servers are good candidates for static assignment.
There are usually so few of these servers that the administrator is not overburdened if a
change in network settings requires going out to reconfigure each individually. Chances
are that the administrator would still need to reconfigure these servers manually (by
using ipconfig /release and then ipconfig /renew ), even if they did not have IP
addresses reserved. Even in large installations, I find it preferable to manage these vital
servers by hand rather than to rely on DHCP.
Reservations are also appropriate for application servers and other special but nonvital
infrastructure servers. With a reservation in DHCP, the client device will still go through
the DHCP process but will always obtain the same addressing information from the DHCP
server. The premise behind this strategy is that these nonvital servers can withstand a
short outage if DHCP settings change or if the DHCP server fails.
Address Pool
The range of IP addresses that the DHCP server can assign is called its address pool . For
example, let's say you set up a new DHCP scope covering the 192.168.1 subnet. That
gives you 255 IP addresses in the pool. After adding an exclusion from 192.168.1.240 to
192.168.1.254, you're left with 241 (255 - 14) IP addresses in the pool. That means (in
theory, at least) that you can service 241 unique clients at a time before you run out of IP
addresses.