The Ethernet frame
Everything a PC sends on a LAN travels inside an Ethernet frame. The IP packet is the letter; the frame is the envelope that gets it across one link.
| Field | Size | What it holds |
|---|---|---|
| Preamble + SFD | 8 bytes | A pattern that lets the receiver lock on and find the start of the frame |
| Destination MAC | 6 bytes | Who the frame is for |
| Source MAC | 6 bytes | Who sent it |
| Type | 2 bytes | What is inside: 0x0800 = IPv4, 0x86DD = IPv6, 0x0806 = ARP |
| Data (payload) | 46 - 1500 bytes | The packet. 1500 bytes is the normal MTU |
| FCS | 4 bytes | A checksum. A frame with a bad FCS is thrown away and counted as a CRC error |
A trunk link between switches adds a 4-byte 802.1Q tag after the source MAC to say which VLAN the frame belongs to (module 8).
MAC addresses
A MAC address is 48 bits, burned into each network card. Cisco writes it as three groups of four hex digits (001a.33d5.62b1), Windows as six pairs with dashes (00-1A-33-D5-62-B1), Linux and macOS with colons (00:1a:33:d5:62:b1). They are all the same address.
| Part | Bits | Meaning |
|---|---|---|
| OUI (Organizationally Unique Identifier) | First 24 | The manufacturer, assigned by the IEEE |
| Device part | Last 24 | Chosen by the manufacturer, unique for each card |
Three kinds of destination
| Kind | Example | Who receives it |
|---|---|---|
| Unicast | 001a.33d5.62b1 | One device |
| Broadcast | ffff.ffff.ffff | Every device in the LAN (the VLAN) |
| Multicast | 0100.5exx.xxxx (IPv4), 3333.xxxx.xxxx (IPv6) | Devices that joined the group |
In NetLab
▶ PC1 NetLab Windows-style command prompt. Type help to see the commands. C:\Users\student>getmac Physical Address Transport Name =================== ========================================================== 00-1A-33-D5-62-B1 \Device\Tcpip_{NetLab} ▶ R1 R1>enable R1#show interfaces g0/0 | include address Hardware is iGbE, address is 001a.6244.3362 (bia 001a.6244.3362) Internet address is 10.1.1.1/24
Remember: A router port has its own MAC address. A switch's access ports also have MACs, but the switch doesn't put them on the frames it forwards: it passes frames on unchanged.