An ETHERNET FRAME looks like:
Ethernet Header --- DATA (Packet) --- Ethernet Trailer
The Ethernet Header contains 5 Fields:
Preamble -- SFD -- Destination -- Source -- Type/Length 7 bytes -- 1 byte -- 6 bytes -- 6 bytes -- 2 bytes
Ethernet Trailer contains 1 Field:
FCS (Frame Check Sequence) = 4 bytes
- The PREAMBLE + SFD is not usually considered part of the ETHERNET HEADER.
THEREFORE the size of the ETHERNET HEADER + TRAILER is 18 bytes
(6 + 6 + 2 + 4 bytes for the FRAME CHECK SEQUENCE)
The MINIMUM size for an ETHERNET FRAME (Header + Payload [PACKET] + Trailer) is 64 BYTES.
64 BYTES - 18 BYTES (Header + Trailer size) = 46 BYTES
THEREFORE the MINIMUM DATA PAYLOAD (PACKET) size is 46 BYTES!
IF the PAYLOAD is LESS than 46 BYTES then PADDING BYTES are added (padding bytes are a series of 0's) until it equals to 46 BYTES.
When a PC sends a packet to a device with an unknown IP address, it uses an ARP Request.
- ARP stands for 'Address Resolution Protocol'.
- It is used to discover the Layer 2 address (MAC address) of a known Layer 3 address (IP address)
- Consists of two messages:
- ARP REQUEST (Source message)
- ARP REPLY (Destination message)
- ARP REQUEST is BROADCAST = sent to all hosts on network, except the one it received the request from.
An ARP REQUEST frame has:
- Source IP Address
- Destination IP Address
- Source MAC address
- BROADCAST MAC Address - FFFF.FFFF.FFFF
An ARP REPLY frame has:
- Source IP Address
- Destination IP Address
- Source MAC address
- Destination MAC Address
ARP REPLY is a known UNICAST frame = Sent only to the host that sent the ARP REQUEST.
PING
- A network utility that is used to test reachability
- Measures round-trip time
- Uses two messages:
- ICMP Echo REQUEST
- ICMP Echo REPLY
- Is UNICAST
- Command to use ping:
- ping
By Default, a CISCO IOS sends 5 ICMP requests/replies (Default size is 100-bytes)
- A period (.) is a failed ping
- An exclamation mark (!) is a successful ping
USEFUL CISCO IOS COMMANDS (from Privileged EXEC mode)
PC1# show arp // shows hosts ARP table
SW1#show mac address-table // show the switches MAC table
Will show:
Vlan --- MAC Address --- Type --- Ports(interfaces)
(Vlan = Virtual Local Area Network)
SW1# clear mac address-table dynamic
// clears the entire switches MAC table. // IF the optional MAC address is used, it will clear the SPECFIC MAC address.
SW1 #clear mac address-table dynamic interface
// clears the MAC table entry of the Switch by it's INTERFACE name.