Cisco CCNA Study Guide - Router Alley

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CCNA Study Guide v2.62 – Aaron Balchunas 80 Class A Subnetting Example Consider the following subnetted Class A network: 10.0.0.0 255.255.248.0 Now consider the following questions: • How many new networks were created? • How many usable hosts are there per network? • What is the full range of the first three networks? By default, the 10.0.0.0 network has a subnet mask of 255.0.0.0. To determine the number of bits stolen: 255.0.0.0: 11111111.00000000.00000000.00000000 255.255.248.0: 11111111.11111111.11111000.00000000 Clearly, 13 bits have been stolen to create the new subnet mask. To calculate the total number of new networks: 2 n = 2 13 = 8192 new networks created There are clearly 11 bits remaining in the host portion of the mask: 2 n – 2 = 2 11 – 2 = 2048 – 2 = 2046 usable hosts per network Calculating the ranges is a bit tricky. Using the shortcut method, subtract the third octet (248) of the subnet mask (255.255.248.0) from 256. 256 – 248 = 8 The first network will begin at 0, again. However, the ranges are spread across multiple octets. The ranges of the first three networks look as follows: Subnet address 10.0.0.0 10.0.8.0 10.0.16.0 Usable Range 10.0.0.1 10.0.7.254 10.0.8.1 10.0.15.254 10.0.16.1 10.0.23.254 Broadcast address 10.0.7.255 10.0.15.255 10.0.23.255 * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
CCNA Study Guide v2.62 – Aaron Balchunas 81 Private vs. Public IPv4 Addresses The rapid growth of the Internet resulted in a shortage of available IPv4 addresses. In response, a specific subset of the IPv4 address space was designated as private, to temporarily alleviate this problem. A public address can be routed on the Internet. Thus, hosts that must be Internet-accessible must be configured with (or reachable by) public addresses. Allocation of public addresses is governed by the Internet Assigned Numbers Authority (IANA). A private address is intended for internal use within a home or organization, and can be freely used by anyone. However, private addresses can never be routed on the Internet. In fact, Internet routers are configured to immediately drop traffic with private addresses. Three private address ranges were defined in RFC 1918, one for each IPv4 class: • Class A - 10.x.x.x /8 • Class B - 172.16.x.x /12 • Class C - 192.168.x.x /24 It is possible to translate between private and public addresses, using Network Address Translation (NAT). NAT allows a host configured with a private address to be stamped with a public address, thus allowing that host to communicate across the Internet. It is also possible to translate multiple privately-addressed hosts to a single public address, which conserves the public address space. NAT provides an additional benefit – hiding the specific addresses and addressing structure of the internal (or private) network. Note: NAT is not restricted to private-to-public address translation, though that is the most common application. NAT can also perform public-to-public address translation, as well as private-to-private address translation. NAT is only a temporarily solution to the address shortage problem. IPv4 will eventually be replaced with IPv6, which supports a vast address space. Both NAT and IPv6 are covered extensively in other guides. * * * All original material copyright © 2013 by Aaron Balchunas (aaron@routeralley.com), unless otherwise noted. All other material copyright © of their respective owners. This material may be copied and used freely, but may not be altered or sold without the expressed written consent of the owner of the above copyright. Updated material may be found at http://www.routeralley.com.
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