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Today when I read a book, there is a sentence:

Data in the Network Access Layer(TCP/IP's layer 1/4), it will be break up into smaller units.

I can not understand this statement, does it mean the IP packets will be divided into several small packets?

I've always thought that the IP packet(layer 2/4) pass to the Network Access Layer(layer 1/4) only add a frame's packet head.

Who can make it clear for me?

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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 packet" networks.

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The internet protocol implements two basic functions: addressing and fragmentation.

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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 functions.

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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.

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  • I have another related doubt: the MTU is located in the OSI's Data Link Layer, we also can say it's location in the TCP/IP( Layer 1/4 ), in your answer does it mean the IP divide fragmentations is the 2/4 layer divide data according to 1/4 layer's standard?
    – aircraft
    Sep 4 '18 at 7:43
  • Yes, the Network Layer must deliver packets small enough that they fit into the payload of the frames for the Data-Link layer. The MTU is the size of the largest Network packet a Data-Link frame for a particular link can hold. If the MTU is smaller than the size of the Network packet being sent, the Network packet must be fragmented according to the IP rules.
    – Ron Maupin
    Sep 4 '18 at 17:16
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Data in the Network Access Layer(TCP/IP's layer 1/4), it will be break up into smaller units.

Because we don't know the context of this statement, its hard to know how it is meant.

I assume the view from an application, which wants to send data through the network. It will give the data to the TCP/IP stack, which will take care of the things needed to transfere the data over the network. Normally application data is much larger as the MTU of a network. So the TCP/IP stack has to cut this data into smaller pieces which then fit into network packets/frames.

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