Three kinds of device
| Device | Its job | It works with |
|---|---|---|
| End device (PC, laptop, server, phone, printer) | Sends and receives the data people actually use | Its own IP address |
| Switch | Joins the devices of one local network (a LAN) together | MAC addresses (hardware addresses) |
| Router | Joins different networks together and picks the path between them | IP addresses |
Think of it like this: A switch is like the corridor inside one office building: it gets a letter from one desk to another desk in the same building. A router is the post office: it takes letters that are going to another building and sends them the right way.
The default gateway
A PC can talk straight to devices in its own network. For anything outside, it hands the packet to the router. That router address is the PC's default gateway. In the network below, PC1 is in 192.168.1.0/24, its gateway is the router at 192.168.1.1, and the server is in a different network, 10.0.0.0/24.
In NetLab
▶ PC1 NetLab Windows-style command prompt. Type help to see the commands. C:\Users\student>ipconfig Windows IP Configuration Ethernet adapter Ethernet: Connection-specific DNS Suffix . : Link-local IPv6 Address . . . . . : fe80::11db:99de:fbce:36d1%12 IPv4 Address. . . . . . . . . . . : 192.168.1.10 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.1.1 C:\Users\student>ping 192.168.1.1 Pinging 192.168.1.1 with 32 bytes of data: Reply from 192.168.1.1: bytes=32 time<1ms TTL=255 Reply from 192.168.1.1: bytes=32 time<1ms TTL=255 Reply from 192.168.1.1: bytes=32 time<1ms TTL=255 Reply from 192.168.1.1: bytes=32 time<1ms TTL=255 Ping statistics for 192.168.1.1: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms C:\Users\student>tracert 10.0.0.10 Tracing route to 10.0.0.10 over a maximum of 30 hops 1 <1 ms <1 ms <1 ms 192.168.1.1 2 * 2 ms 2 ms 10.0.0.10 Trace complete.
ipconfig shows PC1's address and gateway. ping checks it can reach the gateway. tracert lists every router on the way to the server: here the packet goes through the router (192.168.1.1) and then arrives.
How ports are named
Every port has a type and a number. Routers and switches let you shorten the name:
| Full name | Short form | Speed |
|---|---|---|
FastEthernet0/1 | Fa0/1, f0/1 | up to 100 Mb/s |
GigabitEthernet0/0 | Gi0/0, g0/0 | up to 1 Gb/s (1000 Mb/s) |
TenGigabitEthernet1/1 | Te1/1 | up to 10 Gb/s |
The numbers are slot/port: 0/1 means slot 0, port 1. Switches usually count ports from 1; many routers count from 0.
Tip: On the diagram, the small labels next to each cable show which port it plugs into.