How it works
Assigning an address to every device by hand does not scale and invites mistakes. A DHCP service hands out addresses from a pool, along with the other settings a device needs to be useful.
When several devices lose connectivity at once and nothing else changed, suspect the service that supplies their settings before suspecting each device.
Common error
A device that has an address is not necessarily configured correctly. An address without a usable gateway leaves it able to talk locally and nowhere else.
Worked example
Three new clients cannot obtain normal IPv4 configuration. Investigate the address pool.
Evidence
DHCP pool: 10.5.2.100 through 10.5.2.119
Active leases: 20
New clients awaiting leases: 3The inclusive pool contains 20 addresses and is fully leased. Confirm lease utilization and the approved subnet plan before expanding or reclaiming addresses. Also check the supplied mask, gateway, and resolver options; obtaining an address alone does not guarantee a useful configuration.
Exercises
2.3.1
How many addresses are in a DHCP pool from 10.5.2.50 through 10.5.2.79 inclusive? Can it serve 32 simultaneous clients?
Show solution
There are 79 - 50 + 1 = 30 addresses. It cannot supply unique active leases to 32 clients without a larger approved pool or a different allocation plan.
2.3.2
Clients receive valid local addresses but all remote access fails. Which DHCP option should be compared with the network plan first?
Show solution
Inspect the default gateway option and whether that gateway is reachable. A valid local address can support local traffic even when the advertised gateway is missing or wrong.