MAC address
A Medium Access Control (MAC) address is a hardware address used by IEEE 802.3 implementations of Ethernet. Each network device has a unique MAC address. When we send an Ethernet frame, we add our own MAC address in the source field, and the receiver's MAC address in the destination field of the Ethernet header.
The MAC address is 48 bits or 6 bytes in total. We write it in hexadecimal. For example:
- 0000.0c12.3456
Above we have four hexadecimal characters, separated by a dot. There are two other formatting options:
- 00:00:0c:12:34:56
- 00-00-0c-12-34-56
All three refer to the same MAC address. Cisco devices often use the first formatting option, Windows computers use the dash in between.
Normally, a MAC address refers to a single device on the network. We call this a unicast MAC address. There is also a Broadcast MAC address for broadcast traffic (which means that everyone on the network will receive the frame) and Multicast MAC addresses (for a group of receivers).
MAC addresses have to be unique, otherwise, it’s possible that an Ethernet frame ends up at two receivers. To make MAC addresses unique, each networking vendor that wants to build network cards has to ask the IEEE for a unique 24-bit code called the OUI (Organizationally Unique Identifier).
For example, all MAC addresses that start with 0000.0c are owned by Cisco.
The remaining 24 bits of the MAC address are then assigned by the vendor. Each network card that they create will have a unique MAC address. The address that the vendor has assigned is also called the BIA (burned-in address).
When you send a broadcast, the destination MAC address will be FFFF.FFFF.FFFF.
There is also the special case of a MAC address of all zeros.
Links:
Links to this page:
- ARP - Message Header and Payload
- ARP - Static ARP entry for own IP address
- ARP - determining the next hop MAC address
- ARP - promiscuous mode
- ARP table
- ARP
- DHCP - Broadcast flag
- DHCP - Cisco Client ID format
- DHCP - Client ID
- DHCP - How a host is identified by a remote DHCP server
- DHCP - Message Types
- DHCP - Static binding use cases
- DHCP - Using the MAC as the client ID on a Cisco device
- DHCP Request message sent as broadcast
- EtherChannel - LACP System ID
- EtherChannel - Understanding LACP port and system priorities
- EtherChannel - load balancing algorithms
- Ethernet - Pause Frames
- Ethernet header
- HSRP - standby group numbers
- Hardware - Layer-3-aware ASIC and IGMP snooping
- Hot Standby Router Protocol (HSRP)
- ICMP - Mitigating Vulnerabilities
- IPv6 - DHCPv6
- IPv6 - EUI-64 best practice
- IPv6 - NDP Neighbor Discovery Process
- Interface - show interfaces counters explained
- MAC - changing the MAC address on a switchport
- MAC Access List
- MAC Address Table - Static assignment use cases
- MAC address flapping
- MAC address of all zeros
- MAC address table - Switches maintain one table per VLAN
- MAC address table - multiple entries of the same MAC address
- MAC address table - multiple static entries
- MAC address table - show command on Nexus device
- MAC address table - static entry
- MAC address table populated using source address field
- MAC address table static multicast entry
- MAC address table
- Media Access Control (MAC)
- Memory - CAM and TCAM
- Network - BUM Traffic
- Network - Example of communication, encapsulation, and decapsulation, between hosts
- Network - Flapping
- Network - Layer 2 network
- Network - broadcast domain
- Networks - Overlay Transport Virtualization (OTV)
- PTP - Precision Time Protocol
- STP - Aging time reduction with a TCN
- STP - Aging time return to default
- STP - Learning State
- STP - Understanding MST regional root identifier and proposal-agreement process
- STP - port states
- STP Root Bridge selection
- STP Topology Change Timer
- Security - MAC Authentication Bypass
- Security - storm control algorithm
- Switching - CEF Adjacency Table
- Troubleshooting high CPU and memory usage on a switch
- Unknown unicast traffic
- VACL vs ACL
- VLAN Access Lists
- VLAN Tag
- VLANs - when a tagged frame arrives on an access port
- VXLAN - Control Plane Options
- Wake on LAN
- Wireless - Real-time and management functions
- Wireless - WLC authbypass feature