Foundation Summary

The Foundation Summary provides a convenient review of many key concepts in this chapter. If you are already comfortable with the topics in this chapter, this summary might help you recall a few details. If you just read this chapter, this review should help solidify some key facts. If you are doing your final prep before the exam, the following lists and tables are a convenient way to review the day before the exam.

IP addresses are

  • 32-bit numbers (written as four bytes)

  • Divided into a network portion and host portion

Table 2-3 shows the first bits of an IP address, the corresponding classes, and the number of bytes assumed to be in the network portion of the address.

Table 2-3. IP Address Classes
First Bits of IPRange of First ByteClassNetwork Bytes
0 _ _ _ _ _ _ _0–127A1
1 0 _ _ _ _ _ _128–191B2
1 1 0 _ _ _ _ _192–223C3
1 1 1 0 _ _ _ _224–239D—Multicast
1 1 1 1 _ _ _ _240–255E—Experimental

To determine the range of addresses:

If the mask is dotted decimal, convert it to CIDR.

To determine the network address, copy the network bits from the address. Fill in the remaining bits with zeros.

The last address is the broadcast. Copy the network bits and fill in the remaining bits with ones.

The usable addresses fall between these two numbers.

To check, subtract the CIDR length from 32 to determine the number of host bits. There are 2n–2 host addresses.


  • Hides details and protects against flaps

  • Reduces router CPU and memory consumption

  • Saves network capacity

The method for determining the summary is

Step 1.
Write each network in binary.

Step 2.
Determine the number of bits that match. This gives a single summary that includes all the routes.

Step 3.
If step 2 unacceptably over-summarizes, start from the first address and add bits to the prefix until a portion of the range is summarized. Take the remaining addresses and start this process again.