Ccna routing and Switching Complete Study Guide

Internet Protocol (IP)

Internet Protocol (IP) essentially is the Internet layer. The other protocols found here 

merely exist to support it. IP holds the big picture and could be said to “see all,” because 

it’s aware of all the interconnected networks. It can do this because all the machines on the 

network have a software, or logical, address called an IP address, which we’ll explore more 

thoroughly later in this chapter.

For now, understand that IP looks at each packet’s address. Then, using a routing 

table, it decides where a packet is to be sent next, choosing the best path to send it upon. 

The protocols of the Network Access layer at the bottom of the DoD model don’t pos-

sess IP’s enlightened scope of the entire network; they deal only with physical links (local 

networks).

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TCP/IP and the DoD Model 

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Identifying devices on networks requires answering these two questions: Which network 

is it on? And what is its ID on that network? The first answer is the software address, or 

logical address. You can think of this as the part of the address that specifies the correct 

street. The second answer is the hardware address, which goes a step further to specify the 

correct mailbox. All hosts on a network have a logical ID called an IP address. This is the 

software, or logical, address and contains valuable encoded information, greatly simplify-

ing the complex task of routing. (IP is discussed in RFC 791.)

IP receives segments from the Host-to-Host layer and fragments them into datagrams 

(packets) if necessary. IP then reassembles datagrams back into segments on the receiving 

side. Each datagram is assigned the IP address of the sender and that of the recipient. Each 

router or switch (layer 3 device) that receives a datagram makes routing decisions based on 

the packet’s destination IP address.

Figure 3.15 shows an IP header. This will give you a picture of what the IP protocol has 

to go through every time user data that is destined for a remote network is sent from the 

upper layers.

f I g u r e   3 .15     IP header

Total length (16)

Version

(4)

Header

length (4)

Priority and

Type of Service (8)

Header checksum (16)

Time to live (8)

Protocol (8)

Identification (16)

Flags (3)

Fragmented offset (13)

Source IP address (32)

Destination IP address (32)

Options (0 or 32 if any)

Data (varies if any)

Bit 0

Bit 15 Bit 16

Bit 31

20 bytes

The following fields make up the IP header: