--> IP Address operates at Layer 3 of OSI reference model and Layer 2 of TCP/IP model.
--> IP uses Packets to carry the information throughout the network.
--> IP is a connectionless protocol which does not require any acknowledgment from the destination after it has been sent.
1) Version
--> This is the first field in IPv4 Protocol header.
--> The size of this field is of 4 bits.
--> The Version field indicates the current IP version being used(IPv4 or IPv6).
2) Internal Header Length
--> Indicates the size of the IP header.
--> The minimum length of the IP header is 20 bytes and maximum length of 60 bytes.
3) Service Type or Type of Service
--> This field is of 8 bits which are used in Quality of Service.
--> The first three bits of this field are known as precedence bits and are not used currently.
--> The next 4 bits define the type of service and the last bit is left unused.
--> Devices use TOS filed to set various options, such as low delay, high throughput, or high reliability.
4) Packet Length
--> Indicates the size of the packet including IP header.
--> The maximum size of the packet is 65,535 bytes because of the Packet Length field is 16 bits.
--> The minimum size of the packet is 20 bytes.
--> Router performs fragmentation if the size of the packet is more than 65,535 bytes.
5) Identification
--> This field is required when reassembling of IP Packets required.
--> This value is incremented every-time an IP datagram is sent from source to the destination.
--> These fields are used to fragment and reassemble packets.
--> This field is necessary to combine individual IP packets back into a single datagram.
6) Flag
--> The size of this field comprises of three bits.
--> Devices only uses the last two bits of this field as first bit kept reserved.
--> The second bit of this field is known as ‘Don’t Fragment’ bit. If the value of this field is set to 1 then the IP Packet is never fragmented or divided into fragments.
--> The third bit of this field is known as the ‘More Fragment’ bit.If the value of this field is set to 1 then it represents fragmented IP packet and more fragmented IP packets need to come after this.
--> In case of the last fragment of an IP packet, ‘More Fragment’ bit is not set signifying that this is the last fragment of a particular IP datagram.
7) Fragmented offset
--> In case of fragmented IP datagrams, this field contains the offset from the start of IP datagram.
--> So again, this field is used in reassembly of fragmented IP datagrams.
8) Time to live
--> This field represents the number of layer 3 devices or Routers that the IP datagram will go through before being dropped.
--> When the IP Packet arrives at a router, the router decrements the TTL field by one.
--> When the TTL field becomes zero, the router drops the packet by sending an ICMP Time Exceeded message to the sender.
--> The traceroute application uses these ICMP Time Exceeded messages to print the routers used by packets to go from the source to the destination.
9) Protocol
--> The Protocol field defines which application the data is from or which application the data.
--> This field does not identify the application but identifies a protocol that sits above the IP layer that is used for application identification.
--> For example, protocol number 1 = ICMP, 6 = TCP, 17 = UDP.
10) Header checksum
--> Header Checksum contains the value that is calculated based on the data of the IP header.
--> This field is used to determine if any errors have been introduced during the process of sending the IP Packet from source to destination.
--> If the checksum value is same at both sender and receiver then the IP Packet was not corrupted else its assumed that IP Packet was received corrupted.
--> Basically this field is used to check the integrity of an IP Packet.
11) Source IP Address
--> This field is of 32 bits used to define Sender IP Address.
12) Destination IP Address
--> This field is of 32 bits used to define Receiver IP Address.
--> IP uses Packets to carry the information throughout the network.
--> IP is a connectionless protocol which does not require any acknowledgment from the destination after it has been sent.
1) Version
--> This is the first field in IPv4 Protocol header.
--> The size of this field is of 4 bits.
--> The Version field indicates the current IP version being used(IPv4 or IPv6).
2) Internal Header Length
--> Indicates the size of the IP header.
--> The minimum length of the IP header is 20 bytes and maximum length of 60 bytes.
3) Service Type or Type of Service
--> This field is of 8 bits which are used in Quality of Service.
--> The first three bits of this field are known as precedence bits and are not used currently.
--> The next 4 bits define the type of service and the last bit is left unused.
--> Devices use TOS filed to set various options, such as low delay, high throughput, or high reliability.
4) Packet Length
--> Indicates the size of the packet including IP header.
--> The maximum size of the packet is 65,535 bytes because of the Packet Length field is 16 bits.
--> The minimum size of the packet is 20 bytes.
--> Router performs fragmentation if the size of the packet is more than 65,535 bytes.
5) Identification
--> This field is required when reassembling of IP Packets required.
--> This value is incremented every-time an IP datagram is sent from source to the destination.
--> These fields are used to fragment and reassemble packets.
--> This field is necessary to combine individual IP packets back into a single datagram.
6) Flag
--> The size of this field comprises of three bits.
--> Devices only uses the last two bits of this field as first bit kept reserved.
--> The second bit of this field is known as ‘Don’t Fragment’ bit. If the value of this field is set to 1 then the IP Packet is never fragmented or divided into fragments.
--> The third bit of this field is known as the ‘More Fragment’ bit.If the value of this field is set to 1 then it represents fragmented IP packet and more fragmented IP packets need to come after this.
--> In case of the last fragment of an IP packet, ‘More Fragment’ bit is not set signifying that this is the last fragment of a particular IP datagram.
7) Fragmented offset
--> In case of fragmented IP datagrams, this field contains the offset from the start of IP datagram.
--> So again, this field is used in reassembly of fragmented IP datagrams.
8) Time to live
--> This field represents the number of layer 3 devices or Routers that the IP datagram will go through before being dropped.
--> When the IP Packet arrives at a router, the router decrements the TTL field by one.
--> When the TTL field becomes zero, the router drops the packet by sending an ICMP Time Exceeded message to the sender.
--> The traceroute application uses these ICMP Time Exceeded messages to print the routers used by packets to go from the source to the destination.
9) Protocol
--> The Protocol field defines which application the data is from or which application the data.
--> This field does not identify the application but identifies a protocol that sits above the IP layer that is used for application identification.
--> For example, protocol number 1 = ICMP, 6 = TCP, 17 = UDP.
10) Header checksum
--> Header Checksum contains the value that is calculated based on the data of the IP header.
--> This field is used to determine if any errors have been introduced during the process of sending the IP Packet from source to destination.
--> If the checksum value is same at both sender and receiver then the IP Packet was not corrupted else its assumed that IP Packet was received corrupted.
--> Basically this field is used to check the integrity of an IP Packet.
11) Source IP Address
--> This field is of 32 bits used to define Sender IP Address.
12) Destination IP Address
--> This field is of 32 bits used to define Receiver IP Address.
--> This field is of 32 bits used to define Sender IP Address.
13) Options and padding
--> This field varies in length from 0 to a multiple of 32 bits.
--> If the option values are not a multiple of 32 bits, 0s are added or padded to ensure that this field contains a multiple of 32 bits.
--> The options field is not often used. Note that the value in the IHL field must include enough extra 32-bit words to hold all the options.
Reference : Cisco & WikiPedia
Md.Kareemoddin
CCIE # 54759
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