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School/Analyse/IPv4.md

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+# Internet protocol Version 4 (IPV4)
+
+- IP is the principal communications protocol in the network layer.
+- IP has the task of delivering packets from source host to destination host based on IP addresses.
+
+IP defines datagram structure that encapsulate the data to be delivered.
+
+An IP datagram (packet) consists of a header part an a text part. (payload)
+
+The header has a 20 byte fixed part an a variable length optional part.
+
+![[IP-packet.png]]
+
+Some of the fields in a packet are not always used.
+
+## Packet encapsulation
+Packets are encapsulated in data link frames (Ethernet)
+
+![[packet-enscapsulation-2.png]]
+
+## Fragmentation
+Mostly, data link frames are smaller in size than IP Packets
+So the IP Packet does not fit in the data link frame.
+
+In these cases the packet is split up into multiple parts (fragments) and each fragment is send in a separate frame.
+
+![[packet-fragmentation.png]]
+
+The receiver host then combines all fragments before using them
+
+Therefore the receiver should know which part of the message each fragment belongs (hence the fragment offset field in IP header)
+
+![[packet-fragmentation-2.png]]
+
+Ecapsulating a packet in a frame requires:
+
+- Sender MAC Address (can be found out)
+- Receiver MAC Address (Should be found out