Technical definition
The TCP/IP Network Access layer provides the interface between IP and a directly connected network, using the link-specific delivery and physical transmission mechanisms of that network technology. It is also called the Link layer.
In plain English: Groups the local-link framing and physical transmission functions needed to carry an IP packet over a particular network technology.
What this layer does
On Ethernet, a network interface creates a frame with local source and destination MAC addresses, adds an FCS error check and transmits signals over the link. ARP resolves the local IPv4 next-hop address to an Ethernet MAC when needed.
The next link can use a different technology. Not every link uses Ethernet MAC addressing, and not every link has identical framing or reliability features.
The four-layer model combines OSI Data Link and Physical here; a five-layer view separates them.
Follow one request from PC-1
PC-1 is a user's computer opening a page on the lab web server. Its IPv4 address is 192.168.1.20/24 and its gateway is R1 at 192.168.1.1. The server is on another subnet, at 203.0.113.20/24, with gateway 203.0.113.1. The /24 means that the first 24 bits identify each subnet. R1 joins the two subnets.
On small screens, swipe sideways or focus the diagram and use the arrow keys.
In this request: PC-1 sends its remote-server packet inside a frame addressed to R1's local interface. R1 removes that frame, routes the packet and creates a frame for the server-side link. The packet destination and the local frame destination have different jobs.
The interface labels identify the ends of the cables: Eth0 is the example Ethernet interface on each host, while Gi0/0 and Gi0/1 are R1's two Gigabit Ethernet interfaces. Actual device names vary. An IP address identifies an interface; TCP port 443 identifies the HTTPS service endpoint. It is not a physical socket on the router.
Where it fits in the other model
Network Access is also called Link or Network Interface. It corresponds approximately to OSI layers 2 and 1, so this page covers both frame delivery and signals.
The trade-off is between a more detailed vocabulary and a simpler description of deployed protocols. OSI names seven responsibilities; the four-layer TCP/IP model groups some of them together. Neither view requires a separate header for every named responsibility.
When it does not work
If the gateway cannot be resolved, check the active interface, its VLAN and the gateway IP before assuming an application failure. An incomplete ARP entry does not establish why resolution failed. On a switch, a wrong VLAN or a blocked link can prevent local delivery. Growing FCS error counters can point to link or hardware problems; Ethernet detects corrupt frames rather than repairing their contents.
Try a practical check
Use PowerShell on Windows for this observation exercise. These commands inspect your own connection or request a public example service; they do not configure R1 or contact the documentation server in the figure.
Get-NetNeighbor -AddressFamily IPv4
arp -aOn Windows PowerShell, compare IP and link-layer address entries on your active interface. Reachable, stale and incomplete entries describe different cache conditions; a cached MAC does not prove that the peer is responding now. For the diagram's remote destination, the first-hop Ethernet frame uses the local gateway MAC. The actual gateway and MAC on your computer will differ.
Your task: identify one useful field in the result, state what it shows, then name something it cannot prove. If the command is unavailable or a request is blocked, record that limitation rather than treating it as proof of a failed layer.
Check your understanding
PC-1 sends to a server on another subnet. Which destination MAC belongs in its first Ethernet frame?