Which fields of IP header change from router to router?

An IP header is a prefix to an IP packet that contains information about the IP version, length of the packet, source and destination IP addresses, etc. It consists of the following fields:

Here is a description of each field:

• Version – the version of the IP protocol. For IPv4, this field has a value of 4.
• Header length – the length of the header in 32-bit words. The minumum value is 20 bytes, and the maximum value is 60 bytes.
• Priority and Type of Service – specifies how the datagram should be handled. The first 3 bits are the priority bits.
• Total length – the length of the entire packet (header + data). The minimum length is 20 bytes, and the maximum is 65,535 bytes.
• Identification – used to differentiate fragmented packets from different datagrams.
• Flags – used to control or identify fragments.
• Fragmented offset – used for fragmentation and reassembly if the packet is too large to put in a frame.
• Time to live – limits a datagram’s lifetime. If the packet doesn’t get to its destination before the TTL expires, it is discarded.
• Protocol – defines the protocol used in the data portion of the IP datagram. For example, TCP is represented by the number 6 and UDP by 17.
• Header checksum – used for error-checking of the header. If a packet arrives at a router and the router calculates a different checksum than the one specified in this field, the packet will be discarded.
• Source IP address – the IP address of the host that sent the packet.
• Destination IP address – the IP address of the host that should receive the packet.
• Options – used for network testing, debugging, security, and more. This field is usually empty.

Consider the following IP header, captured with Wireshark:

Notice the fields in the header: the IP version is IPv4, the header length is 20 bytes, the upper-level protocol used is TCP, the TTL value is set tu 128, source and destination IP addresses are listed, etc.

Download our Free CCNA Study Guide PDF for complete notes on all the CCNA 200-301 exam topics in one book.

We recommend the Cisco CCNA Gold Bootcamp as your main CCNA training course. It’s the highest rated Cisco course online with an average rating of 4.8 from over 30,000 public reviews and is the gold standard in CCNA training:

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education

So, since both source and destination are in the same network/subnet, so the switch will handle this traffic and do switching which is processing the headers by checking the DST MAC and decide which port to send the traffic out (without anything change in the header).

2- PC1 Communicating with PC08 (Called Routing / or Remote Communication)

The packet/traffic goes out for PC1 will have this in the header,

SRC MAC : AAA

DST MAC : CCC

SRC IP :10.0.0.1

DST IP :80.0.0.8

TCP Header (as per the application or traffic type), and will never get changed at any stage.

This time is different, the source and destination are in different network/subnet, so the PC decide to send this traffic to its gateway which is the router (bcz from PC perspective, the gateway is only device in the network who should know about the way/path to reach any unknown networks)

that's why you see that DST MAC includes the Mac of the gateway, by sending such packet with such header, the switch receives it and knows where to send it, out of the ports connected to the router (switch can't understand the IP header, only deals with MACs).

So far, no changes in the header at all. 

But, once the R receives the packet, it will consult its routing table and figure the path/route to network 80.0.0.0 which should be through R2.

Here you are, R1 change the SRC MAC R1-Mac and DST and R2-MAC, and keep the rest of the header as the same.

Then, R2 will do the same, change the SRC MAC to R2-MAC and DST MAC to R3-MAC...an d so on.

In conclusion, IP headers remains the same throughout the whole journey from source to destination, but the MAC addresses changes hop-by-hop.

**** Please rate this post as helpful if it helped you, and as a correct answer if it solved your problem. ****

Which one of the following fields of an IP header is not modified by the router?

Which fields of an IP packet changes from router to router?

Which field in the IP header is decremented by every router it passes through?

What does IP header modify by an IP router?