• In the TCP/UDP header, the source and the destination port number will be specified.
• In the IP header, the source and the destination IP address will be specified.
• How data is transferred from transport layer to physical layer is hidden by the OS.

• The size of the IP header is 20 bytes (4 bytes per line x 5 lines)
• IP datagram length doesn't include header size.
• upper layer protocol= tcp or udp
• If TTL is 20, after each hop on the router, the TTL decrements by 1, and when it reaches 0, the router it is located at will discard the packet to prevent the packet from staying alive forever.
• header checksum checks bit error of the IP header only - the TCP / UDP checksum checks the bit error in the data.

• MTU is the maximum amount of data a link-level frame can carry.
• If the MTU on the receiving network is smaller, then the IP datagrams are fragmented to fit the MTU standard.
  • ex) 1200 bytes is fragmented to 3 smaller datagrams.
• The fragments are later reassembled.
  
• In all fragments, ID remains unchanged.
• offset is the first byte being sent divided by 8.
  • ex) 480 is being sent, offset is 60.
• flag is 1 if there is next fragment to come.
• flag is 0 if this is the last fragment.

• The 32 bits IP address is soon to be exhausted.

ICMP

• Used for error troubleshooting.
• ICMP header starts after IP header.

• ping is used to check if destination is responsive.
  
• traceroute is used to trace all the routers that the packet goes through.