DHCP

What this chapter covers

DHCP gives hosts their IP settings automatically, and ARP lets them find each other once they have them.

The chapter covers the RouterOS DHCP client and server, the options a server hands out, and how to manage leases. It then covers ARP, the RouterOS ARP table and the ARP mode of an interface.

Both topics sit between the physical network and the routing layer. They decide whether a host can talk on its local network at all, so a fault here looks like a total outage even when routing is fine.

How the topics fit together

The two topics meet at the point where a new host joins a LAN.

A new host first asks for an address, and the DHCP server answers from its pool. The reply also includes the gateway and DNS server from the network entry.

Once the host has an address, it uses ARP to learn the MAC addresses of its neighbours and of the gateway. The router can tie the two together, because the DHCP server can create ARP entries for its leases.

The practical questions in this chapter follow that story:

• Which interface does the router listen on, and does it have an address in the served subnet?
• Which addresses are in the pool, and which belong to static leases?
• Which gateway and DNS server does the network entry give to clients?
• Does the ARP mode of the interface allow the host to be resolved at all?

How to use these pages

Each topic page has the same shape, so you can skim for what you need.

• What it is: the idea in plain words.
• How it works: the mechanism, with diagrams and console examples.
• What to remember for the exam: the facts to memorise.
• Common mistakes: the classic traps.
• Check yourself: three questions to test your understanding.

Build a small lab with one router and two hosts. Set up a DHCP server, watch the lease appear, and then change the ARP mode to see what happens to the hosts. Seeing the failure for yourself makes the exam wording easier to remember.