Unit6_LAN - Unit 6 LAN Technologies LAN Technologies 6-1 In...

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Unit 6 LAN Technologies LAN Technologies 6-1
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In this unit, we will focus on IEEE 802.3 Ethernet. Note that LLC is the upper sublayer while MAC is the lower sublayer. From the viewpoint of LLC, there is a point-to-point link between sender and receiver. LAN Technologies 6-2
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Unit 6: Outline 6.1 Link-Layer Addressing 6.2 Ethernet 6.3 Link-Layer Switches 6.4 Spanning Tree Protocol 6.5 VLAN LAN Technologies 6-3
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Unit 6.1 Link-Layer Addressing LAN Technologies 6-4
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MAC Addresses 32-bit IP address: network-layer address used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address: link-layer address function: get frame from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable LAN Technologies 6-5
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Why MAC address? If there is IP address, why do we need to have MAC address? Protocol Stack: Separation of Functionality Each layer has its own address Link layer devices (e.g. NIC) do not understand IP. IP was not the only network-layer protocol. IPX: a popular networking protocol from late 1980s to mid-1990s. AppleTalk: the primary protocol used by Apple devices during 1980s and 90s. LAN Technologies 6-6
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LAN Addresses Each adapter on LAN has unique LAN address Broadcast address = FF-FF-FF-FF-FF-FF = adapter 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN (wired or wireless) Hexadecimal notation LAN Technologies 6-7
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LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) analogy: (a) MAC address: like HKID card number (b) IP address: like postal address MAC flat address portability can move LAN card from one LAN to another IP hierarchical address NOT portable address depends on IP subnet to which a node is attached LAN Technologies 6-8
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ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 237.196.7.23 237.196.7.78 237.196.7.14 237.196.7.88 LAN Technologies 6-9
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ARP Table Each IP node (Host, Router) has an ARP table ARP Table: IP/LAN address mappings for some LAN nodes Example: IP address LAN address TTL 137.189.97.61 88-B2-2F-54-1A-34 13:45:00 137.189.97.63 5C-66-A4-23-C3-23 13:50:00 TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) LAN Technologies 6-10
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How to construct ARP Table? LAN broadcast address: FF-FF-FF-FF-FF-FF is used to send ARP request LAN Technologies 6-11
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ARP protocol: Same LAN (network) A wants to send datagram to B, and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address dest MAC address = FF- FF-FF-FF-FF-FF all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to- MAC address pair in its ARP table until information
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