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For a normal wired connection, plug both computers into the router’s LAN ports, or connect a switch to the router and plug both computers into the 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 local network tries to reach a local service using the router’s public IP address.
Choose the wiring that matches what you want to do
“One wire” can mean a direct cable between two computers, a single uplink cable from a switch to a router, or a test of a local service through the router’s public address. These are different network arrangements.
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- For ordinary local networking: connect both computers to the router’s LAN or to a switch connected to it.
- For extra Ethernet ports: connect a switch to a router LAN port, then connect each computer to the switch.
- For a direct computer-to-computer link: connect the computers to each other, but expect to configure the network if it does not work automatically.
- For testing a local service by public IP: the wiring may already be correct; the router’s NAT loopback behavior may be the issue.
Connect both computers through the router or a switch
The usual layout is router LAN → switch (if needed) → both computers. If the router has enough working LAN ports, connect each computer directly to one. A switch is useful when you need more Ethernet ports; it does not replace the router’s addressing, firewall, or internet-connection functions.
Google’s guidance for supported Nest and Google Wi-Fi equipment describes a switch as a way to add Ethernet ports and says it lets devices communicate and share the internet connection: Add Ethernet ports to Nest Wifi Pro, Nest Wifi, or Google Wifi. Ubuntu’s general wired-network guidance likewise recommends connecting two computers through a hub, router, or switch rather than assuming a direct cable will create a ready-to-use network: Connect to a wired (Ethernet) network.
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Use one connection from the switch to the router’s LAN, then attach the computers to the switch. Do not add a second active cable between the same network segments unless the equipment is configured to prevent loops. Multiple active paths can cause slow or erratic connectivity, or a network failure; see NETGEAR’s explanation of network loops.
When a direct cable between two computers makes sense
A direct Ethernet link can be useful when you specifically want the two hosts connected to each other without a router or switch. It is not the same as joining both computers to a shared LAN: the isolated cable may not provide DHCP, the service that commonly assigns network addresses. Each computer needs a compatible link and an address on a network they can both use. The exact setup depends on the operating systems, network adapters, and whether either computer is configured to provide address assignment or routing.
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Do not assume a crossover cable is always required. Older setup guidance sometimes called for crossover cabling between directly connected hosts, but requirements depend on the equipment and its link-negotiation support. Cisco’s troubleshooting guidance covers cable suitability and condition, including the historical crossover consideration: Troubleshooting TCP/IP.
What hairpin NAT does—and what it does not do
Computers on the same LAN can normally reach one another using their local network addresses. Their ordinary local traffic does not need to leave through the router’s public-facing NAT path.
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Hairpin NAT, also called NAT loopback, describes a different case: a device inside the LAN tries to reach a service hosted on another local machine by using the router’s public IP address. The router must send that request back into the LAN to the internal server. MikroTik defines the behavior in those terms in its RouterOS NAT documentation.
Whether this works depends on the router model and firmware, and sometimes on its configuration. Consult the router’s manual for supported behavior and instructions rather than assuming every router has the same toggle or setup. A switch adds physical ports; it does not enable hairpin NAT. An alternative in some networks is split DNS, where the service name resolves to a local address for clients inside the LAN, but whether that is practical depends on the router or DNS setup.
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Troubleshoot in the order that isolates the fault
- Identify the test: decide whether both computers should share the router/switch LAN, are connected directly to each other, or whether a LAN client is accessing a local service through the public IP.
- Check the physical link: reseat the cable, look for link indicators at the ports, and try a known-good cable or port. Confirm both computers are powered on. HP’s wired-network checklist also calls out cable connections, hub lights, the other computer’s power, and firewall settings: Troubleshoot wired home networks in Windows.
- Check each computer’s address: confirm both interfaces have addresses on the network you intend to use. On a direct isolated cable, do not assume a router is providing DHCP.
- Test the local address first: reach the service using the server computer’s LAN address. If that works but reaching it by public IP from inside the LAN fails, investigate NAT loopback or split DNS instead of replacing the cable or switch.
- Check the host and service: verify the computer’s firewall allows the connection and that the service is listening on the expected interface and port. A cable or switch cannot correct a blocked firewall rule or a service bound only to another address.
- Verify switch wiring: use a single uplink from the switch to the router LAN, with the computers connected to the switch. Remove redundant paths unless managed loop prevention is configured.
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