Fix IPv4 Exhaustion: Shorten DHCP Lease or Expand Subnet

You've got a device that won't connect, and the error says something like 'No IP address available' or 'IPv4 address exhaustion detected.' I've seen this a hundred times. It's annoying, but it's almost always fixable in a few minutes.

The Quick Fix: Shrink the DHCP Lease Time

First thing: log into your router's admin panel. Usually that's 192.168.1.1 or 192.168.0.1. Look for DHCP settings. Find the lease time. It's probably set to 1440 minutes (24 hours) or even a week. Change it to 360 minutes (6 hours). Save and reboot the router.

DHCP Lease Time: 1440 minutes → 360 minutes

This frees up IP addresses faster when devices go to sleep or disconnect. It's not a perfect solution—your network still has the same pool size—but it stops new devices from failing immediately.

Why This Happens

Your router hands out IP addresses from a fixed range. Usually that's 192.168.1.2 to 192.168.1.254. That's 253 addresses. If you've got 30 devices on Wi-Fi, plus guests, plus IoT stuff, you can hit that limit fast. Once the pool is full, the router can't give out any more IPs. The lease time makes it worse—those old devices hog addresses for hours or days even after they leave.

The culprit here is almost always a combination of too many devices and a lease time that's way too long. Shortening the lease time is the fastest fix. But it's a band-aid.

The Real Fix: Expand the Subnet

If you have more than 50 devices regularly, or if you're running a business network, change the subnet mask. Go back to your router's LAN settings. Change the subnet mask from 255.255.255.0 (a /24) to 255.255.254.0 (a /23). This doubles your usable IPs from 253 to 509.

Old: 192.168.1.0/24 → 253 usable IPs
New: 192.168.1.0/23 → 509 usable IPs

You'll also need to update the DHCP pool to match. Set start address to 192.168.0.2 and end to 192.168.1.254. (Yes, that's a bigger range, but most routers handle it fine.) Some older routers might not let you use a /23 mask. If yours won't, you're stuck with the quick fix or need a new router.

Less Common Variations

Sometimes the issue isn't the pool size but the DHCP server itself. I've seen Windows Server DHCP run out of addresses because someone set a reserved range that overlapped with the pool. Check your exclusion ranges. Or you might have a rogue DHCP server handing out wrong IPs. Use ipconfig /all on a client and look for the DHCP server IP. If it's not your router, shut it down.

Another odd one: stale ARP cache on a managed switch. Unlikely on home gear, but on a Cisco or HP switch, clear the ARP cache with clear arp. This forces the switch to re-learn which IPs are active.

Also, check if you've got any static IPs set manually on devices. If you assign a static IP that's inside the DHCP pool, the router might give that address to another device. Conflict. Fix: use static IPs outside the DHCP range, like 192.168.1.200-254.

Prevention

Set a reasonable lease time. 360 minutes for home, 240 minutes for busy offices. Don't set it to 7 days unless you have a very small network.
Use a /23 subnet if your router supports it. That's the biggest single change that prevents this from coming back.
Run a DHCP audit once a year. Use a tool like dhcplist.exe or just look at the router's DHCP client list. Remove old devices that don't exist anymore.
Consider a separate VLAN for IoT devices if you have more than 30 of them. Most consumer routers won't do this, but if you're running Ubiquiti or Omada, split your network. IoT on a /24, main devices on another /24. Solves congestion and security.

Bottom line: if you see that exhaustion error, drop the lease time first. Then plan to expand the subnet. Don't waste time rebooting devices or resetting the router—that only clears the pool for a few minutes. Do it right, and you won't see this message again for years.