CCENT 2 S02 E07 – Basic TCP/IP: The Tale of Two Packets

  CCENT, Networking, TCP/IP

Bob Packet – The “Local Boy”

privatenetworktraffic

Bob is a ‘PING’ request that will travel from 172.10.1.20 to 172.10.1.30.  Subnet Mask is 255.255.255.0

  • .20 Compares its Network ID (172.10.1) with that of the Destination IP (172.10.1) and sees they are a match.
  • .20 sends an ARP (Address Resolution Protocol) asking for the MAC address of the computer with the .30 IP.
  • ** Non-matching computers will simply drop the packet. **
  • .30 sees it is looking for its MAC, and sends it back to .20  “Hi, My IP address is 172.10.1.30 and my MAC address is AA00:BC33:3211”
  • Armed with the MAC address of the Destination IP, it sends Bob Packet to the switch.
  • The Switch knows which “Switchport” the Destination MAC address is connected to and automatically sends Bob to the correct computer.
  • Bob looks something like this:  [PING][sPORT][dPORT][sIP=.20][dIP=.30][sMAC=:3923][dMAC=:3211]

 

Sally, The Adventuring Packet

publicinternettraffic

 

Sally is a PING request that will travel from 172.10.1.20 to 192.168.0.100.  Subnet Mask = 255.255.255.0

  • .20 compares the Network ID with the Destination IP with its own and it does NOT match, so it knows it must go through the Default Gateway.
  • .20 sends out an ARP for the Default Gateway, 172.10.1.1 and will get its MAC address :3212
  • Sally is sent: [SALLY][sPORT][dPORT][sIP=172.10.1.20][dIP=192.168.0.100][sMAC=:3923][dMAC=:2301]
  • The Router (R1) checks the Destination IP and knows to sent it through.  Since the Destination Network ID is not connected to it, it looks at its Rounting Table and sees it should go next to 10.10.1.2.
  • Sally is sent to R2: [SALLY][sPORT][dPORT][sIP=172.10.1.20][dIP=192.168.0.100][sMAC=:2301][dMAC=:3DAA]
  • Repeat past 2 steps until Sally is sent to R4, :3210  [SALLY][sPORT][dPORT][sIP=172.10.1.20][dIP=192.168.0.100][sMAC=:AABB][dMAC=:3210]
  • R4 receives Sally and recognizes the Network ID.  It then sends an ARP to get the correct MAC for the IP.
  • Once the MAC for the Destination IP is determined, Sally is sent to the correct server.
  • Finally Sally:  [SALLY][sPORT][dPORT][sIP=172.10.1.20][dIP=192.168.0.100][sMAC=:3211][dMAC=:0399]

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