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 exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

Remember the following important points regarding IP addressing:

  • The IP address is 32 bits long.

  • The network/host boundary can be anywhere in the 32 bits.

  • The Internet allocates a unique bit pattern. These bits are the first bits on the far left and are not available for you to use for networks because they identify your organization to the Internet.

  • The Internet authority ARIN (in the United States) will provide the left (network) portion of the address to the organization to use for addressing within their network. The bits provided on the right portion of the address are zeroed and are allocated by the organization as subnet and host addresses.

  • The network mask is the identification of the bits allocated to the network, defined on all participating routers.

CIDR solves the problem of the excessive network resources required to manage the huge routing tables. Table 2-7 and Table 2-8 include further quick reference CIDR material.

Table 2-7. RFCs about CIDR

RFC Number



Applicability Statement for the Implementation of Classless InterDomain Routing (CIDR)


An Architecture for IP Address Allocation with CIDR


Classless Interdomain Routing (CIDR) : An Address Assignment and Aggregation Strategy


Exchanging Routing Information Across Provider Boundaries in the CIDR Environment

Table 2-8. Table to Illustrate the Use of Prefix Masks



New Address Space


12 percent of Class C

30 hosts


24 percent of Class C

62 hosts


50 percent of Class C

126 hosts


2 Class Cs

510 hosts


4 Class Cs

1022 hosts


8 Class Cs

2046 hosts


16 Class Cs

4094 hosts

Table 2-9 shows an example of an IP address in both decimal and binary format.

Table 2-9. An IP Address and Mask Shown in Binary


Octet 1

Octet 2

Octet 3

Octet 4

IANA address in decimal




IANA address in binary





Prefix as a subnet mask in decimal




Prefix as a subnet mask in binary





If it were a standard Class C address, the mask would be By making the mask, the last three bits of the third octet are essentially giving the organization eight Class C networks.

The following main rules apply when subnetting:

  • A subnet can be used to address hosts, or it can be used for further subnetting.

  • All 1s or all 0s in the subnet portion of the classful network cannot be used. If this rule has been followed, any subnet that is further subnetted does not need to obey this rule, as it has already been observed .

  • The routing protocol must carry the subnet mask in its updates.

  • Multiple IP addresses intended to be summarized must have the same high-order bits.

  • Routing decisions are made on the entire address, preferring the longest bit pattern available.

The two main reasons for using VLSM are as follows :

  • To make efficient use of the available addressing

  • To enforce a good hierarchical design, allowing summarization and documentation

The advantages of summarization are as follows:

  • To reduce the size of the routing table.

  • To reduce network overhead.

  • To make communication of routing updates more efficient.

  • To reduce CPU and memory utilization.

  • To simplify management.

  • To maximize the use of IP addresses by allowing a more granular application of addresses. Thus, a point-to-point link can be given two host addresses, while a switched LAN has 254 host addresses available. Otherwise, the point-to-point link would be assigned the equivalent of a Class C address.

  • To isolate topographical changes from other areas.

CCNP BSCI Exam Certification Guide
CCNP BSCI Exam Certification Guide (CCNP Self-Study, 642-801) (3rd Edition)
ISBN: 1587200856
EAN: 2147483647
Year: 2002
Pages: 194
Authors: Clare Gough © 2008-2017.
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