IP DATAGRAM SIZE:
Datagrams may have different sizes i.e. Header area is generally fixed (20 octets) but can have various options. Data area may contain between 1 octet and 65.535 octets (216-1).Generally, data area is much bigger than header.
Header contains all information required to deliver datagram to the receiver computer. It has:
Router checks header of every datagram and forwards datagram along path to destination.