For ordinary wired networking, connect both computers to the router’s LAN ports—or connect a switch to the router and plug both computers into that switch. A direct cable between the computers is a different setup and may need manual network configuration. Hairpin NAT is relevant only when a computer on your LAN tries to reach a local service by using the router’s public IP address.
Choose the right wiring for two computers
Use the router or a switch for a shared LAN
The usual arrangement is router LAN → computer A and computer B. If the router does not have enough LAN ports, connect a switch to one of its LAN ports, then connect both computers to the switch:
Router LAN → switch → computer A and computer B
On supported Nest and Google Wifi equipment, Google describes a switch as a way to add Ethernet ports and says it lets devices communicate and share the internet connection. See Google’s instructions for adding Ethernet ports. If your router has enough working LAN ports, you do not need a switch.
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Connect the computers directly only when that is your goal
A cable can connect two computers directly, but that does not automatically give them the same network setup as computers connected through a router. Each network interface needs a usable address on the same network, and the link must negotiate successfully. On an isolated direct connection, do not assume a router is present to provide DHCP addresses. The exact setup depends on the operating systems and network adapters.
Ubuntu’s wired-network guidance recommends connecting two computers through a hub, router, or switch and notes that a direct cable may require additional setup: Connect to a wired (Ethernet) network. Some older guidance calls for crossover cabling in certain direct connections; Cisco’s troubleshooting material includes that historical caveat, but it should not be treated as a universal requirement for modern equipment: Cisco TCP/IP troubleshooting.
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What hairpin NAT does—and when it matters
Computers on the same LAN can communicate using their local, or LAN, addresses. They do not ordinarily need to send that traffic through the router’s public-facing NAT path.
Hairpin NAT, also called NAT loopback, addresses a different situation: a device inside the LAN tries to reach a service hosted on another device inside that LAN by using the router’s public IP address. The router must send the request back into the local network. MikroTik defines the feature in these terms in its RouterOS NAT documentation.
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Support and setup vary by router model and firmware. Consult your router’s documentation rather than expecting a particular setting name or a universal configuration recipe. If a service works from another device using its LAN address but fails when you use the public address from inside the home, investigate NAT loopback or split DNS before changing cables or adding a switch.
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Troubleshoot the problem in this order
- Identify what you are trying to connect. Is each computer meant to join the router/switch LAN, are you making a direct computer-to-computer link, or are you testing a local service through the router’s public address?
- Check the physical link. Reseat the cable, check port or link lights, and try a known-good cable and port. HP’s wired-network checklist also recommends checking that connected equipment is powered on and reviewing firewall settings: Troubleshoot wired home networks in Windows.
- Check network addresses. Confirm that both interfaces have addresses appropriate to the intended network. For an isolated direct cable, verify that each computer has a usable address; do not assume DHCP is available without a router or other DHCP server on that link.
- Test the service by its LAN address. If that works but access through the public address from inside the LAN does not, check router NAT-loopback support or consider split DNS.
- Check the host and service. A computer’s firewall can block connections, and an application may not be listening on the network interface or port you expect.
- Wire an added switch correctly. Use one connection from the switch to the router’s LAN, then connect the computers to the switch. Avoid creating multiple active paths between the same network segments unless the equipment is configured to prevent loops; network loops can cause slow or erratic connectivity, or a loss of service. See NETGEAR’s explanation of network loops.
What a cable or switch can—and cannot—fix
- A switch adds Ethernet ports when the router’s available LAN ports are insufficient.
- A known-good cable or port can help identify a physical-link fault.
- Neither a switch nor a cable inherently fixes incorrect IP addressing, firewall rules, a service bound to the wrong interface, DNS, or unsupported hairpin NAT.
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