MAC

• The MAC address is the physical address of a device
• Devices need the MAC address for communication on a local area network
• Devices use ARP to acquire the MAC address for a device

-> An IP address is used to locate a device on a network -> A MAC address is what identifies the actual device

MAC flat address: portability

• can move interface from one LAN to another
• recall IP address not portable: depends on IP subnet to which node is attached

Addressing ARP

ARP (Address Resolution Protocol) -> Used to Resolve IP addresses to MAC addresses

• Maintained by hosts and routers.
• In order to find a MAC address, computer A searches it's internal list, called an ARP cache, if computer's B IP address already has a matching MAC address
• The ARP cache is used to make a network more efficient, storing IP address to MAC address associations
• Each host/router has it's own ARP cache
• The ARP cache stores <IP address; MAC address; TTL>

-> arp -a -> Checks ARP cache

• If there is no match, computer A sends a broadcast message out on the network asking each device which computer matches the respective IP address and ask for the specific MAC address
• The computer that matches the IP address will send it's MAC address to computer A
• This info will be stored in computer A's ARP cache
• TTL(Time To Live): time after which address mapping will be forgotten (typically 20 min)

ARP entries can be:

• Dynamic -> created automatically when a device sends a brodcast message to the network; Are not permanent, flushed out periodically
• Static -> created manually, entered using ARP command utilities;

Ethernet

• “dominant” wired LAN technology
• simpler, cheap
• single chip, multiple speeds
• bus (90s): all nodes in same collision domain (can collide with each other)
• switched(prevaile today): layer-2, nodes do not collide with each other

→ connectionless: no handshaking between sending and receiving NICs → unreliable: receiving NIC doesn’t send ACKs or NAKs to sending NIC. Data in dropped frames recovered only if initial sender uses higher layer

• Ethernet’s MAC protocol: unslotted CSMA/CD with binary backoff
• many different Ethernet standards, different physical layer media: fiber, cable

Ethernet Switch

• link-layer

• examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment

• transparent: hosts unaware of presence of switches

• switches do not need to be configured

• Ethernet protocol used on each incoming link, so: -> no collisions; full duplex -> each link is its own collision domain

• A can communicate with A' and B to B' without collisions

• but A-to-A’ and C to A’ can not happen simultaneously

• Maintained by Layer 2 switches. Each switch has a switch table, each entry:

• (MAC address of host, interface to reach host, time stamp)

• looks like a routing table!

• when frame received, switch “learns” location of sender: incoming LAN segment

• records sender/location pair in switch table

VLANs - Virtual LANs

• switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure

• port-based VLAN: switch ports grouped (by switch management software) so that single physical switch operates as multiple virtual switches. -> traffic isolation: frames to/from ports 1-8 can only reach ports 1-8 -> dynamic membership: ports can be dynamically assigned among VLANs -> forwarding between VLANS: done via routing (just as with separate switches)
• trunk port: carries frames between VLANS defined over multiple physical switches
• VLANS can communicate through multiple switches

• In a VLAN a computers, servers and other network devices are logically connected regardless of their physical location

• Improve security, traffic management and simplifies a network

• Lets suppose there is a building with 3 floors, and each floor has mixed computers that belong to different departments (accounting, shipping, support)

• They are all connected to the same switch in a LAN

• All network broadcast traffic are mixed in with other departments

• VLANS can logically create several virtual networks to separate the network broadcast traffic

• The traffic between the 3 departments are isolated, so they can't see each others traffic even though they all share the same cabling and switch

• This is achieved by assigning specif ports in the switch to specific VLAN

• It helps with traffic management cause as the network grows the frequency of the broadcast will also increase congestion in the network
• With VLANS we separate things into smaller broadcast domains helping alleviate the overall broadcast traffic