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Fundamentals of IP for the CCNA INTRO Exam #640-821

Chapter Description

Learn about the core concepts behind OSI Layer 3 and the main Layer 3 protocol used by TCP/IP to help you on the CCNA INTRO Exam.

Foundation Summary

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your CCNA exam, a well-prepared CCNA candidate should know, at a minimum, all the details in each "Foundation Summary" section before going to take the exam.

The routing process forwards the packet, and only the packet, from end to end through the network, discarding data-link headers and trailers along the way. The network layer processes deliver the packet end to end, using successive data-link headers and trailers just to get the packet to the next router or host in the path. Figure 5-15 shows the concepts behind encapsulation used by routers.

Figure 15Figure 5-15 Network Layer and Data Link Layer Encapsulation

Table 5-8 outlines several Layer 3 address structures.

Table 5-8 Layer 3 Address Structures


Size of Address in Bits

Name and Size of Grouping Field in Bits

Name and Size of Local Address Field in Bits



Network or subnet (variable, between 8 and 30 bits)

Host (variable, between 2 and 24 bits)



Network (32)

Node (48)



Network* (16)

Node (8)



Many formats, many sizes

Domain-specific part (DSP—typically 56, including NSAP)

*Consecutively numbered values in this field can be combined into one group, called a cable range.

The general ideas about how IP address groupings can be summarized as follows:

  • All IP addresses in the same group must not be separated by a router.

  • IP addresses separated by a router must be in different groups.

Table 5-9 summarizes the characteristics of Class A, B, and C networks.

Table 5-9 Sizes of Network and Host Parts of IP Addresses with No Subnetting

Any Network of This Class

Number of Network Bytes (Bits)

Number of Host Bytes (Bits)

Number of Addresses per Network*


1 (8)

3 (24)

224 – 2


2 (16)

2 (16)

216 – 2


3 (24)

1 (8)

28 – 2

*There are two reserved host addresses per network.

Network numbers look like actual addresses because they are in dotted-decimal format. However, network numbers are not actually IP addresses because they cannot be assigned to an interface as an IP address.

Table 5-10 summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network.

Table 5-10 List of All Possible Valid Network Numbers*


First Octet Range

Valid Network Numbers*

Total Number of This Class of Network

Number of Hosts per Network


1 to 126 to

27 – 2

224 – 2


128 to 191 to

214 – 2

216 – 2


192 to 223 to

221 – 2

28 – 2

*The Valid Network Numbers column shows actual network numbers. There are several reserved cases. For example, networks (originally defined for use as a broadcast address) and (still available for use as the loopback address) are reserved. Networks,,, and also are reserved.

When subnetting, the host part of the address shrinks to make room for the subnet part of the address. Figure 5-16 shows the format of addresses when subnetting.

Figure 16Figure 5-16 Address Formats When Subnetting Is Used

The goals described in the following list are common for any IP routing protocol, regardless of its underlying logic type:

  • To dynamically learn and fill the routing table with a route to all subnets in the network.

  • If more than one route to a subnet is available, to place the best route in the routing table.

  • To notice when routes in the table are no longer valid, and to remove those routes from the routing table.

  • If a route is removed from the routing table and another route through another neighboring router is available, to add the route to the routing table. (Many people view this goal and the preceding one as a single goal.)

  • To add new routes, or to replace lost routes with the best currently available route, as quickly as possible. The time between losing the route and finding a working replacement route is called convergence time.

  • To prevent routing loops.