One method is to limit datagram size to smallest MTU of any server. IP needs fragmentation i.e. datagrams can be divided into pieces to fit in network with small MTU. Router checks datagram larger than network MTU and then it divided into pieces and every piece is smaller than outbound server MTU.
Each fragment is an independent datagram packet. It adds all header fields. Bit in header shows that the datagram is a fragment. Other fields have information for reconstructing original datagram packet. Fragment offset provides original location of fragment. Router has local MTU to device size of every fragment. It puts part of datagram from original datagram in every fragment and puts other information into message header. The fragmentation process is given in the diagram below.