What Is DHCP? Address Leases, Reservations and Checks

DHCP lets a client obtain network configuration from a service instead of entering every setting manually. That service may run inside a home router or on separate enterprise servers. Selecting automatic addressing does not guarantee Internet access: the gateway, DNS settings and network attachment still have to be correct.

What DHCPv4 supplies

DHCPv4 can provide an IPv4 address, subnet mask, default gateway and DNS servers. The lease describes the address allocation and its lifetime. A misconfigured scope or option can distribute incorrect settings just as efficiently as correct ones. Microsoft's DHCP overview describes centralized configuration and address management.

This explanation focuses on DHCPv4. IPv6 is not a copy using the same exchange and ports: SLAAC, Router Advertisements and DHCPv6 have distinct roles. In particular, IPv6 default-router discovery uses Router Advertisements, so looking only for a DHCPv6 server is incomplete troubleshooting. The IPv6 Neighbor Discovery specification defines that discovery process.

Obtaining a lease is different from renewing it

The familiar initial exchange is Discover, Offer, Request and ACK: find a server, receive an offer, request the chosen allocation and receive confirmation. DHCPv4 uses server UDP port 67 and client UDP port 68. Renewal does not necessarily repeat all four messages; a client normally attempts it before expiry. RFC 2131 specifies these exchanges.

There is no universal 24-hour lease rule. Inspect the server configuration and the lifetime actually assigned to the client. Renewing the same address is normal; dynamic configuration does not promise a new address every time the device reconnects.

Reservations are still automatic client configuration

With a reservation, the client continues using DHCP while the server maps a matching client to a selected address. A manually configured static address is set on the client instead. Assigning arbitrary static addresses inside an unmanaged DHCP pool can cause collisions. Multiple servers can provide planned redundancy; uncoordinated overlapping allocations are the problem.

If a reservation fails to match, inspect the active MAC address and any client identifier used by the server. A reservation for Ethernet does not automatically cover the same laptop's wireless adapter.

Read the configuration before changing it

On Windows, inspect the relevant adapter:

ipconfig /all

Review DHCP enablement, server, lease times, gateway and DNS together. On Linux, these commands display addresses and default routes; they do not by themselves identify the DHCP server or prove a valid lease:

ip -br address
ip route show default

Use the installed network manager for lease details. A 169.254.x.x address on a client expecting automatic IPv4 configuration can suggest failed DHCP acquisition, but is not conclusive on its own. Check link and VLAN membership, available pool addresses and server logs. If addressing is valid but only name resolution fails, investigate DNS separately.

Across subnet boundaries

Initial broadcasts do not automatically cross a router into every other network. A server on another subnet commonly requires a correctly configured DHCP relay and a suitable scope. Wrong VLAN membership, relay settings or filtering can prevent a client from receiving an answer even while the server is healthy.

Do not run ipconfig /release casually on a remotely managed machine: releasing its address can terminate your session. Review the release and renew documentation and arrange local access or a recovery path first. Save the current lease evidence, then verify the expected address, gateway and DNS values after any deliberate change.