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ARP

ARP (Address Resolution Protocol) is a network protocol used to find out the hardware (MAC) address of a device from an IP address. It is used when a device wants to communicate with some other device on a local network (for example on an Ethernet network that requires physical addresses to be known before sending packets). The sending device uses ARP to translate IP addresses to MAC addresses. The device sends an ARP request message containing the IP address of the receiving device. All devices on a local network segment see the message, but only the device that has that IP address responds with the ARP reply message containing its MAC address. The sending device now has enough information to send the packet to the receiving device.
 
ARP request packets are sent to the broadcast addresses (FF:FF:FF:FF:FF:FF for the Ethernet broadcasts and 255.255.255.255 for the IP broadcast).
 
ARP broadcast explained: 
 
arp process
 
Let’s say that Host A wants to communicate with host B. Host A knows the IP address of host B, but it doesn’t know the host B’s MAC address. In order to find out the MAC address of host B, host A sends an ARP request, listing the host B’s IP address as the destination IP address and the MAC address of FF:FF:FF:FF:FF:FF (Ethernet broadcast). Switch will forward the frame out all interfaces (except the incoming interface). Each device on the segment will receive the packet, but because the destination IP address is host B’s IP address, only host B will reply with the ARP reply packet, listing its MAC address. Host A now has enough information to send the traffic to host B.
 
All operating systems maintain ARP caches that are checked before sending an ARP request message. You can display ARP entries in Windows by using the arp -a command:
 
windows arp
 
Jan 26, 2016upravnik
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CONTENT #1
  • Networking basics
    • What is a network?
    • OSI & TCP/IP models
    • Encapsulation
    • Ethernet
    • Ethernet frame
    • MAC & IP addresses
    • Unicast, multicast, and broadcast addresses
    • Network devices
    • Half duplex and full duplex
    • IEEE Ethernet standards
    • Cisco three-layer hierarchical model
  • Cabling
    • Types of Ethernet cabling
    • Types of Ethernet cables
  • Types of networks
    • Wide area network
    • Local area network & Metropolitan area network
  • IP addressing
    • Types of IP addresses
    • Classes of IP addresses
    • Subnetting explained
    • Subnet mask
    • Create subnets
  • Network tools
    • Ping
    • Traceroute
  • Network protocols
    • TCP/IP suite of protocols
    • TCP explained
    • UDP explained
    • Ports explained
    • ARP
    • DHCP & DNS
    • Telnet & SSH
    • FTP & TFTP
    • SNMP (Simple Network Management Protocol)
    • HTTP & HTTPS
    • NTP (Network Time Protocol)
    • APIPA (Automatic Private IP Addressing)
    • ICMP (Internet Control Message Protocol)
    • IP header
  • Cisco IOS
    • Cisco IOS overview
    • Power on a Cisco device
    • IOS command modes
    • Get help in IOS
    • Running & startup configuration
    • IOS basic commands
    • Configure descriptions
    • Run privileged commands within global config mode
    • Ports on an IOS device
    • Pipe character in IOS
    • IOS boot sequence
    • Backing up IOS configuration
  • IP routing
    • What is IP routing?
    • Connected, static & dynamic routes
    • Administrative distance & metric
    • Routing protocols
  • RIP
    • RIP overview
    • Configuring RIPv2
    • RIP loop prevention
  • EIGRP
    • EIGRP overview
    • EIGRP configuration
    • EIGRP automatic & manual summarization
    • EIGRP authentication & load balancing
    • EIGRP summary
  • OSPF
    • OSPF overview
    • OSPF configuration
    • Designated & Backup Designated Router
    • OSPF authentication
    • OSPF summarization
    • OSPF summary
    • Differences between OSPF and EIGRP
  • LAN switching
    • Layer 2 switching
    • Collision & broadcast domain
    • CSMA/CD
  • VLAN
    • What is a VLAN?
    • Configuring VLANs
    • Configuring access & trunk ports
    • Frame tagging
    • IEEE 802.1Q
    • Inter-Switch Link (ISL)
CONTENT #2
  • ACLs
    • What are ACLs?
    • Types of ACLs
    • Configuring standard ACLs
    • Configuring extended ACLs
  • NAT
    • What is NAT?
    • Static NAT
    • Dynamic NAT
  • IPv6
    • What is IPv6?
    • IPv6 address format
    • Types of IPv6 addresses
    • IPv6 unicast addresses
    • IPv6 global unicast addresses
    • IPv6 unique local addresses
    • IPv6 link-local addresses
    • IPv6 multicast addresses
    • IPv6 address prefixes
    • IPv6 interface identifier
    • IPv6 transition options
    • IPv6 routing protocols
    • How to configure IPv6
    • RIPng
    • Differences between IPv4 and IPv6
  • Miscellaneous
    • Wildcard masks
    • Setting up Telnet
    • Setting up SSH
    • Port security
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