Technical definition
The TCP/IP Internet layer provides connectionless, best-effort packet delivery across interconnected networks using IP addressing and forwarding. IPv4 and IPv6 are its principal network protocols.
In plain English: Uses IP addresses and packet forwarding to carry traffic between networks.
What this layer does
The Internet layer includes IPv4 and IPv6. Each router examines the packet destination and selects a next hop using its routing information. A matching default route is used only when no more specific route takes precedence.
IP is a best-effort service: it does not set up a reliable end-to-end connection before forwarding each packet. Lost data, duplicate packets and out-of-order arrival must be handled where the application and transport protocols require it.
Internet is the layer's name; IP also works inside a private network with no public internet connection.
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: R1 receives a packet addressed to 203.0.113.20. Its connected route to 203.0.113.0/24 selects Gi0/1. It decreases the IPv4 TTL, a limit on how many router hops a packet can survive, and builds an outgoing Ethernet frame. This example uses no NAT.
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
The Internet layer corresponds to OSI layer 3, Network. Its job is distinct from the Ethernet or Wi-Fi delivery used on each individual link.
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
A wrong subnet mask can make the host treat a remote address as local. A wrong gateway can stop traffic to other networks while local communication still works. Read the address and prefix together, then check the destination's most specific matching route. The default route is a fallback; a more specific route can take precedence. A routing table entry is evidence of a chosen path, not proof that every device on it forwards successfully.
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.
ipconfig
route print -4On Windows, first identify your active interface's IPv4 address, mask and gateway. Then inspect the IPv4 route table. Network Destination and Netmask describe a route's prefix; Gateway and Interface show where it goes next. Your real addresses will differ from this lab. Do not replace working settings with the documentation addresses shown in the diagram.
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
A PC has address 192.168.1.20. Can you determine its subnet from that address alone?