DHCP and DNS on a Company Network: Concepts and Common Mistakes

DHCP and DNS are two services users never hear about, but when either one has a problem, users see it as “the network is down.” Understanding these two services makes a large share of everyday network problems understandable and quick to fix.

What Does DHCP Do?

DHCP automatically gives every device that connects to the network its network settings: an IP address, subnet mask, default gateway and DNS server addresses. A few key concepts:

• Scope: the range of addresses DHCP is allowed to assign to devices.
• Lease: an address is assigned to a device for a set period, and the device renews it before it expires.
• Reservation: a specific address is always assigned to a particular device, based on its MAC address; useful for printers and devices that need a fixed address.
• Exclusion: addresses within the scope that DHCP should not assign to anyone, because they are set manually on equipment.

What Does DNS Do?

DNS translates names into IP addresses, both for internet sites and for the company’s internal servers. On a network with Active Directory, DNS plays an even more critical role: computers find the domain controller through DNS records. That’s why:

• Domain-joined computers should use only internal DNS, meaning the domain controllers.
• Internal DNS forwards requests for internet names to external DNS through a Forwarder.

Common Mistakes and Their Symptoms

1. A 169.254.x.x address: if a computer has an address like this, it received no response from any DHCP server and assigned itself a temporary address. Check the cable, the switch port, the port’s VLAN or the DHCP service itself.
2. Rogue DHCP: someone has connected a home router or modem to the network, and that device is also handing out addresses. Some users get the wrong address and cannot reach the internet or servers. The DHCP Snooping feature on managed switches prevents this problem.
3. Address exhaustion: on a guest network or busy Wi-Fi, if the lease time is long, addresses run out and new devices cannot get one. Use shorter lease times for these networks.
4. Address conflicts: a static address set manually on a device falls inside the DHCP scope and is also assigned to another device. Place static addresses outside the scope or in the exclusion list.
5. Internet DNS on domain computers: user logins become slow, Group Policy does not apply, or the computer cannot find the domain.
6. Stale DNS records: a server’s name still points to its old address. Configure automatic cleanup of stale records (Scavenging) carefully.

If you have only one DHCP server and it fails, devices are left without addresses once their leases expire. On Windows Server, you can enable DHCP Failover between two servers.

Useful Troubleshooting Commands

• ipconfig /all: shows the computer’s address, gateway, DNS and DHCP server.
• ipconfig /release followed by ipconfig /renew: releases the current address and gets a fresh one from DHCP.
• ipconfig /flushdns: clears the computer’s DNS cache.
• nslookup: checks which address a name resolves to and which DNS server responds.

Checklist

• The DHCP scope does not overlap with static addresses.
• The correct gateway and DNS are set in the DHCP settings.
• A backup DHCP server or Failover is in place.
• DHCP Snooping is enabled on managed switches.
• Domain computers use only internal DNS, and a Forwarder is configured.
• Reservations and scopes are documented.

If your users struggle with strange connection drops or slow domain logins, ITPlus724 engineers can review and fix your network’s DHCP and DNS settings.