What is the difference between DHCP scope options and server options?

Server options apply to all scopes on the DHCP server and serve as defaults. Scope options apply only to a specific scope (subnet) and override server options when both are configured. Use server options for settings common across your network (like DNS servers) and scope options for subnet-specific settings (like different default gateways per VLAN).

Which DHCP options are needed for PXE boot?

PXE boot typically requires Option 066 (Boot Server Host Name) to specify the TFTP/WDS server address, and Option 067 (Bootfile Name) to specify the network boot program path. Option 060 (PXE Client) can also be used to identify PXE clients. In environments with DHCP and WDS on the same server, you may need to configure IP Helper addresses instead.

Should I use DHCP Option 004 or Option 042 for time synchronization?

Use Option 042 (NTP Servers) for time synchronization. Option 004 references the older TIME/ITP protocol (RFC 868), which is not recommended. Option 042 specifies NTP/SNTP servers (RFC 1769), which is the modern standard. However, in Active Directory environments, domain-joined machines sync time through the AD hierarchy automatically.

Configuring DHCP Options: Learned lessons

When configuring a DHCP server, most administrators set up the basics — an IP address pool, a subnet mask, and perhaps a default gateway. But DHCP supports dozens of options that can streamline network configuration and reduce manual work on client machines. This guide covers the most commonly used DHCP options, the difference between scope and server-level options, PXE boot configuration, and troubleshooting strategies.

Commonly Used DHCP Options

DHCP options are numbered fields defined by IANA that carry configuration parameters from the server to the client. Below are the options you will encounter most frequently.

Option 003 — Router (Default Gateway)

Option 003 tells the client which IP address to use as its default gateway. This is arguably the most critical option after the IP address and subnet mask themselves.

Option 003: Router
Value: 192.168.1.1

You can specify multiple gateway addresses, but clients typically only use the first one. If you have redundant gateways (for example with HSRP or VRRP), point this option at the virtual IP rather than a single physical router.

Option 006 — DNS Servers

Option 006 provides the client with one or more DNS server addresses. Most environments specify two or three DNS servers for redundancy.

Option 006: DNS Servers
Value: 10.0.1.10, 10.0.1.11

The order matters. Clients will try the first server and fall back to subsequent servers only when the primary is unreachable (not when it returns NXDOMAIN). Place your fastest or most reliable DNS server first.

Option 015 — DNS Domain Name

Option 015 sets the DNS suffix on the client. This allows users to type short hostnames (like fileserver) instead of fully qualified domain names (like fileserver.corp.example.com).

Option 015: DNS Domain Name
Value: corp.example.com

This option configures the primary DNS suffix for DHCP clients. For domain-joined Windows machines, the domain suffix is typically set through Group Policy, but this option is important for non-domain-joined devices.

Option 044 — WINS/NBNS Servers

Option 044 specifies the NetBIOS Name Service (WINS) servers. While WINS is considered legacy, many environments still rely on it for older applications or backward compatibility.

Option 044: WINS/NBNS Servers
Value: 10.0.1.20

If your environment has fully migrated away from NetBIOS name resolution, you can skip this option. However, removing it prematurely can break older applications that depend on WINS.

Option 046 — WINS/NBT Node Type

Option 046 is often configured alongside Option 044. It controls how NetBIOS name resolution behaves on the client.

Value	Node Type	Behavior
0x1	B-node	Broadcast only
0x2	P-node	Point-to-point (WINS only)
0x4	M-node	Mixed (broadcast first, then WINS)
0x8	H-node	Hybrid (WINS first, then broadcast)

The recommended setting is 0x8 (H-node), which queries WINS first and falls back to broadcast. This reduces unnecessary broadcast traffic on the network.

DHCP Options for Network Time Servers

When working with servers, having a reliable time source to keep all systems synchronized is important. DHCP offers two time-related options that can be confusing.

Option 004 — Time Server

Option 004 specifies servers that provide TIME/ITP as per RFC 868. This is the older time protocol and is not recommended in modern environments.

Option 042 — NTP Servers

Option 042 specifies servers that provide NTP/SNTP as per RFC 1769. This is the preferred time service and the one you should use.

Option 042: NTP Servers
Value: 10.0.1.10

In Active Directory environments, domain-joined machines automatically synchronize time through the AD hierarchy (clients sync with their authenticating DC, DCs sync with the PDC Emulator). Option 042 is most useful for non-domain-joined devices like Linux servers, network appliances, or IoT devices.

Scope Options vs. Server Options

Understanding the hierarchy of DHCP options is essential to avoid misconfigurations.

Server-Level Options

Server options apply as defaults across all scopes managed by the DHCP server. Configure settings here that are consistent network-wide:

DNS servers (Option 006) — usually the same for all subnets
DNS domain name (Option 015) — typically one domain across the organization
WINS servers (Option 044) — if used, generally the same everywhere

Scope-Level Options

Scope options apply to a specific scope (subnet) and override any server-level option with the same number. Use these for settings that vary by subnet:

Default gateway (Option 003) — different for each VLAN/subnet
NTP servers (Option 042) — if subnets have local NTP servers

Reservation-Level Options

You can also set options at the reservation level for individual clients. These override both scope and server options.

Priority order (highest to lowest):

Reservation-level options
Scope-level options
Server-level options

A common mistake is configuring the default gateway at the server level. Since each subnet has a different gateway, Option 003 should always be set at the scope level.

PXE Boot Options (060, 066, 067)

PXE (Preboot Execution Environment) allows computers to boot from the network, which is essential for OS deployment with tools like WDS (Windows Deployment Services) or SCCM.

Option 060 — PXE Client

Option 060 identifies the vendor class. PXE clients send PXEClient as their vendor class identifier. You can use this to create vendor-specific option classes on the DHCP server.

Option 066 — Boot Server Host Name

Option 066 specifies the TFTP server or WDS server address that PXE clients should contact to download the boot file.

Option 066: Boot Server Host Name
Value: 10.0.1.50

Option 067 — Bootfile Name

Option 067 specifies the path to the network boot program on the TFTP server.

Option 067: Bootfile Name
Value: boot\x64\wdsnbp.com

For UEFI vs. BIOS boot, you may need to configure different boot files. Modern setups use DHCP policies or vendor classes to serve the correct boot file based on the client architecture:

BIOS clients: boot\x86\wdsnbp.com
UEFI clients: boot\x64\wdsmgfw.efi

If your DHCP server and WDS server are on the same machine, do not configure Options 066 and 067 in DHCP. Instead, configure WDS to respond to PXE requests directly through the WDS console:

Open the WDS console
Right-click the server and select Properties
On the DHCP tab, check Do not listen on DHCP ports and Configure DHCP options for Proxied DHCP

Common DHCP Configuration Mistakes

Avoid these frequent errors when configuring DHCP options:

Setting the gateway at the server level — The default gateway (Option 003) must be set per scope since each subnet has its own router interface.
Forgetting to authorize the DHCP server in AD — In an Active Directory environment, the DHCP server must be authorized before it will serve leases.
Overlapping scopes — Ensure no two scopes on different DHCP servers hand out the same IP ranges unless you are using DHCP failover with split scopes.
Wrong subnet mask — Double-check the subnet mask matches your network design. A mismatch causes clients to be unable to reach resources on other subnets.
Configuring PXE options when WDS is colocated — As noted above, if WDS and DHCP share the same server, let WDS handle PXE responses rather than setting Options 066/067 in DHCP.

Troubleshooting DHCP Issues

When DHCP clients are not receiving the correct configuration, work through these steps:

Check the Current Client Configuration

On Windows clients, run:

ipconfig /all

Look for the DHCP Server field and verify the options received (DNS servers, default gateway, domain suffix).

Release and Renew the Lease

ipconfig /release
ipconfig /renew

Inspect the DHCP Server Logs

DHCP server logs are located at %windir%\System32\dhcp. The log files are named by day of the week (for example, DhcpSrvLog-Mon.log). Look for NACK entries or scope exhaustion warnings.

Verify Scope Statistics

In the DHCP console, right-click a scope and select Display Statistics. Check the percentage of addresses in use. If the scope is more than 80% utilized, consider expanding it.

Test with a Specific DHCP Request

Use dhclient on Linux or a packet capture tool like Wireshark to examine the DHCP DORA (Discover, Offer, Request, Acknowledge) process and verify which options are included in the server’s response.

# On Linux, request a lease and display verbose output
sudo dhclient -v eth0

Verify IP Helper Addresses

If clients on a remote subnet are not receiving DHCP offers, ensure the router or layer-3 switch has an IP helper address configured pointing to the DHCP server:

interface Vlan100
  ip helper-address 10.0.1.5

Summary

DHCP options extend far beyond basic IP assignment. By understanding the commonly used options — Router (003), DNS (006), Domain Name (015), WINS (044), and NTP (042) — along with PXE boot options (060, 066, 067), you can automate network configuration across your entire environment. Always configure options at the appropriate level (server, scope, or reservation), and use the troubleshooting steps above when clients do not receive the expected settings.