One of the functions of IP is fragmentation and reassembly of packets too large to traverse the network.
From RFC 791, Internet Protocol:
The internet protocol also provides for fragmentation and reassembly
of long datagrams, if necessary, for transmission through "small
The internet protocol implements two basic functions: addressing and
The internet modules use fields in the internet header to fragment and
reassemble internet datagrams when necessary for transmission through
"small packet" networks.
The model of operation is that an internet module resides in each host
engaged in internet communication and in each gateway that
interconnects networks. These modules share common rules for
interpreting address fields and for fragmenting and assembling
internet datagrams. In addition, these modules (especially in
gateways) have procedures for making routing decisions and other
In the routing of messages from one internet module to another,
datagrams may need to traverse a network whose maximum packet size is
smaller than the size of the datagram. To overcome this difficulty, a
fragmentation mechanism is provided in the internet protocol.