Router not assigning IP addresses

Troubleshooting: Router Not Assigning IP Addresses

When client systems connect to a network but fail to obtain an IP configuration, it indicates a failure in local IP address lease negotiation. This guide details the diagnostics and resolution steps for DHCP service outages.

How DHCP Works (The DORA Process)

The Dynamic Host Configuration Protocol (DHCP) is a client-server protocol that automates IP parameter allocation. It operates on UDP Port 67 (server) and UDP Port 68 (client) through a four-way handshake:

1. 1Discover: The client broadcasts a DHCP Discover packet to locate active DHCP servers on the subnet.
2. 2Offer: The router DHCP server reserves an IP from its pool and broadcasts or unicasts a DHCP Offer containing the proposed IP, subnet mask, gateway, and DNS servers.
3. 3Request: The client broadcasts a DHCP Request confirming it accepts the offered IP configuration.
4. 4Acknowledgement (ACK): The server registers the client's MAC address, binds the IP lease, and sends a DHCP ACK to finalize the configuration.

Common Failure Points:

• Discover packets are dropped due to network loops (STP blocks) or VLAN isolation.
• The DHCP daemon crashes or exhausts memory buffers, dropping Discover broadcasts.
• The router fails to send an ACK because the IP lease is claimed by another static device.

DHCP Lease Lifecycle

The allocation of dynamic IP addresses follows a specific state-transition cycle:

| State | Trigger | Action | | --- | --- | --- | | INIT | Client network interface active | Client sends DHCP Discover broadcast | | SELECTING | Client receives DHCP Offer(s) | Client selects best offer and prepares Request | | REQUESTING | Client sends DHCP Request | Client requests lease confirmation from server | | BOUND | Client receives DHCP ACK | Lease active; client binds IP configuration | | RENEWING (T1) | Lease time reaches 50% | Client unicasts DHCP Request to renew lease | | REBINDING (T2) | Lease time reaches 87.5% | Client broadcasts DHCP Request to any DHCP server | | EXPIRED | Lease time reaches 100% | IP released; interface disabled; INIT state triggered |

How To Verify DHCP Using Command Line

#### Windows CMD / PowerShell Run these commands in an elevated Command Prompt or PowerShell terminal:

• Display Full Configuration: ipconfig /all - verify if your adapter has a DHCP enabled flag and check your current gateway and DNS.
• Release Current Lease: ipconfig /release - forces the system to drop its current IP allocation.
• Request New Lease: ipconfig /renew - triggers a new DORA handshake to obtain fresh configuration parameters.

#### macOS Terminal Locate your active interface (usually en0 or en1) and run:

• Renew DHCP Lease: sudo ipconfig set en0 DHCP - releases and requests a new DHCP config.
• Query Configuration: networksetup -getinfo "Wi-Fi" or networksetup -getinfo "Belkin USB-C LAN" - inspects lease info.

#### Linux Terminal Use the standard DHCP client daemon dhclient or NetworkManager's command line interface nmcli:

• Release and Renew using dhclient:

  sudo dhclient -r eth0
  sudo dhclient eth0

• Toggle interface using nmcli:

  nmcli device disconnect eth0
  nmcli device connect eth0

DHCP Failure Causes vs Solution Matrix

| Observed Fault | Root Cause | Resolution Steps | | --- | --- | --- | | APIPA Fallback (169.254.x.x) | No DHCP Offer received; server down or link blocked | Restart DHCP service; verify physical cabling | | IP Lease Depletion | High volume of guest connections or smart devices | Extend DHCP pool range; shorten lease duration | | Rogue DHCP Server | Conflict from active DHCP on extender/AP | Log into secondary AP and toggle DHCP to Disabled | | VLAN Handshake Failure | Discover packets blocked at boundary | Configure DHCP Relay / IP Helper on Layer 3 switch | | STP Broadcast Loop | Mesh node switch loops flood CPU | Enable Spanning Tree Protocol (STP) on local switches |

Static IP vs DHCP

• DHCP (Dynamic Host Configuration Protocol): The router automatically assigns IP parameters from a dynamic pool. Addresses can change when the lease expires, which is perfect for general clients but problematic for hosts needing consistent routes.
• Static IP Configuration: Manually binding an IP address directly to the client network adapter. This bypasses the DHCP server entirely.
• Risk of Collisions: If you configure a static IP manually (e.g., 192.168.1.150) and that address lies inside the router's active DHCP range, the router may assign it to another device, triggering an IP address conflict error and dropping both devices offline. To prevent this, always set static IP reservations in the router GUI under DHCP Reservations / Static Lease bindings.

DHCP Pool Exhaustion

A router's address pool defines the beginning and end of assignable IPs (e.g., 192.168.1.100 to 192.168.1.199, supporting 100 devices). When guests, smart home devices, and mobile random-MAC devices connect, they lease these addresses.

• NAT Mapping Limits: If the pool is fully leased, the DHCP server ignores Discover packets. Device number 101 will fail to get an IP, causing an APIPA fallback.
• Lease Duration Settings: By default, many routers set lease durations to 24 or 72 hours. To avoid exhaustion on high-traffic networks, reduce the lease duration to 8 or 12 hours. This instructs the router to reclaim inactive IP addresses faster.

Internal References

• If your device is configured but you can't access the login screen, see the [router-login](/router-login) checklist.
• For instructions on setting up base configuration properties, read our guide on [router-settings](/router-settings).
• If your WAN link is active but you receive DNS resolving warnings, look at [dns-server-not-responding](/dns-server-not-responding) or [default-gateway-not-available](/default-gateway-not-available).
• To clear all system states and reset the DHCP daemon, check [router-reset](/router-reset).