What is DHCP?

Dynamic Host Configuration Protocol (DHCP) is a client-server protocol that automatically assigns IP addresses and other network settings – such as the default gateway and DNS servers – to devices from a configured address pool. It removes the need to configure each device’s network settings by hand, and it is the default way devices join IP networks.

Key takeaways

• DHCP automatically assigns IP addresses and settings to clients.
• A client-server protocol; a relay agent forwards requests across subnets.
• DHCPv4 and DHCPv6 use lease, options, and address-pool profiles.
• Leases expire and renew on a timer, reclaiming unused addresses.
• VOS devices can act as a local DHCP server or a DHCP relay agent.

On this page: Why it matters · How it works · Types · DHCP & Versa · FAQ

Why is DHCP important?

Every device that joins a network needs an IP address, a subnet mask, a default gateway, and usually a DNS server – getting any of these wrong breaks connectivity or creates a conflict. Before DHCP, administrators assigned these settings by hand on every host, which did not scale and was a constant source of typos and duplicate addresses.

DHCP centralizes address management. A device broadcasts a request when it connects, and a server answers with an address from a defined pool and the rest of its configuration, without an administrator touching the endpoint. This matters most where device counts are large and unpredictable – branch offices, guest Wi-Fi, BYOD and IoT – where hosts appear and disappear constantly. Leases expire and are reclaimed automatically, and a changed DHCP option propagates to every client at its next renewal.

What is DHCP?

DHCP works through a client-server exchange, sometimes summarized as DORA: a client broadcasts a Discover message, a server responds with an Offer of an available address, the client sends a Request confirming it, and the server replies with an Acknowledgment that finalizes the lease. The exchange typically completes in a few packets before the client uses the network at all.

Address assignment is governed by profiles. A lease profile sets how long an address stays valid and when the client must renew it. An options profile defines the parameters delivered with the address, such as the default router, DNS servers, and vendor-specific options. When a DHCP client and server sit on different subnets, a DHCP relay agent forwards messages between them, since discovery broadcasts do not cross routed boundaries on their own.

Types of DHCP

DHCP splits along a few axes. DHCPv4 and DHCPv6 serve IPv4 and IPv6 networks respectively; DHCPv6 can run stateful, handing out addresses and parameters together, or stateless, supplying only configuration such as DNS servers while addresses are assigned another way.

By role, a device can be a DHCP server, allocating addresses from its own pool, or a relay agent, forwarding requests to a server on a different subnet without allocating anything itself. By allocation method, a server can hand out addresses dynamically, reclaiming them at lease expiry, or statically, mapping a MAC address to the same IP every time – useful for printers and other devices needing a predictable address.

DHCP & Versa

VOS devices can be configured as a local DHCP server, a DHCP relay agent, or both, built from profiles – lease, options, static mapping, and address pool – associated with a server or, at the global DHCP level, used as the default when a server does not override them. Match conditions let a server or relay agent select which requests it answers, by interface, client identifier, or hardware address; ping-based checking avoids offering an address already in use.

On LAN profiles, DHCP server and relay settings are configured directly on IRB interfaces alongside VLAN settings, and CSG appliances configure the same options through Versa Portal. Lease state is stored locally and synchronizes between devices in active-standby HA pairs.

Frequently asked questions

What is the DHCP DORA process?

DORA stands for Discover, Offer, Request, and Acknowledge – the four messages exchanged when a client obtains an address. The client broadcasts a Discover, a server replies with an Offer, the client confirms with a Request, and the server finalizes with an Acknowledgment.

What is the difference between a DHCP server and a DHCP relay agent?

A DHCP server allocates IP addresses from a configured pool and answers requests itself. A relay agent does not allocate addresses; it forwards DHCP messages between clients and a server on a different subnet, necessary because discovery relies on broadcasts that routers do not forward.

What is the difference between DHCPv4 and DHCPv6?

DHCPv4 assigns IPv4 addresses and configuration to clients. DHCPv6 serves IPv6 networks and can run statefully, assigning addresses and parameters together, or statelessly, supplying only configuration such as DNS servers while addresses come from IPv6 stateless autoconfiguration instead of the server.

What happens when a DHCP lease expires?

Each assigned address has a lease lifetime; before it expires, the client sends a renewal request, and the server extends the lease if the address is still valid. If a client never renews and the lease lapses, the server reclaims the address for reassignment.

Can DHCP assign the same IP address to a device every time?

Yes. A static mapping profile binds a device’s MAC address to a fixed IP address, so the DHCP server always offers that address to the same device instead of pulling one from the dynamic pool – useful for printers, servers, and other infrastructure that needs a predictable address.

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