4.3Basic routing concepts

Many faults of the form "I reach people in the same room but only the outside server is unreachable" are about the exit to another network. Every time, a PC decides: if the destination is on its own network (the local network), hand it directly to the peer; if on a different network (a remote network), hand it to the default gateway, the "exit to the outside." This section covers the role of that exit and the difference between an L2 switch (handling within one segment) and an L3 switch (which can also route between networks), as the introductory basis for telling "does this destination go directly or via the gateway."

4.3.1Default gateway and local/remote

• A local network destination is on the same network (same subnet) as you; it is reached directly by switching within one segment, without a router. A remote network destination is on a different network and cannot be reached directly—it must go through the "exit to the outside."
• The default gateway is the address of the router (or L3 switch) that serves as the exit to hand a packet to when the destination is on another (remote) network. A PC first decides "is the destination on my own network?"; if not, it sends to the default gateway, and from there the router routes it to the final destination.
• So a missing or wrong default gateway produces the classic symptom of "communicating within the same network but unable to leave for the outside (the internet or another subnet)." The standard move is to check the gateway value with ipconfig (Windows).

4.3.2L2 switch vs. L3 switch

• An L2 switch forwards frames within one segment (the same network) by looking at MAC addresses. It does not make cross-network decisions (routing); packets bound for another network are left to the device acting as the default gateway. The ordinary "switch" is this.
• An L3 switch adds to fast L2 switching the ability to route between networks by looking at IP addresses. Its benefit is serving as the exit that interconnects multiple internal networks (VLANs)—the default-gateway role—in a single box. It performs a router-like role at high speed within the LAN.
• The knack for telling them apart: "is the destination on my own network (reached by L2 switching) or another network (routing needed)?" Grasp the division—reaching within one network is switching, reaching another network is routing.

Suppose a user reports: "I can open the shared folder at the next desk, but only a server in another department and the internet are completely unreachable." A layer-and-routing view lets you pinpoint the cause quickly. First, "I reach the next desk (the same network, the local network)" means this PC's physical connection and the switch's switching within one segment are working. The problem is that "only another department and the internet (a different network, a remote network) are unreachable," the classic pattern where the exit to another network—the default gateway—is suspect. Checking this PC's IP address and default gateway with ipconfig (Windows) often reveals the gateway address blank or off by one from the correct value. A PC first decides "is the destination on my own network?"; if yes it switches directly, if no it hands off to the default gateway—so a missing gateway produces exactly this "inside OK, outside NG" symptom. Note that as the exit bundling multiple internal networks (VLANs), an L3 switch often performs that routing role (the default gateway) in one box at high speed, so knowing "which device is the gateway" from the diagram speeds isolation. At the introductory stage, more than complex route calculation, being able to clearly draw the line "local is direct, remote is via the gateway" is the key to solving this kind of trouble.

4.3.3Section summary

• A local network (same subnet) destination is switched directly; a remote network one is routed via the default gateway
• An L2 switch forwards within one segment by MAC (no routing); an L3 switch can also route between networks by IP
• "Inside works but only the outside is unreachable" is the classic missing/wrong default-gateway symptom—check the setting with ipconfig