CCENT/CCNA ICND1 100-101 Official Cert Guide

An Example of Adding a New VLSM Subnet

For example, Figure 20-3 shows an existing internetwork that uses VLSM. (The figure uses the same IP addresses as shown in Figure 20-2, but with R3’s LAN IP address changed to fix the VLSM overlap shown in Figure 20-2.) In this case, you need to add a new subnet to support 300 hosts. Imagine that the question tells you to use the smallest subnet (least number of hosts) to meet that requirement. You use some math and logic you learned earlier in your study to choose mask /23, which gives you 9 host bits, for 2⁹ – 2 = 510 hosts in the subnet.

Figure 20-3 Internetwork to Which You Need to Add a /23 Subnet, ...