Chapter 1: Network Overview

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If an address is set to all 1s, it applies to all addresses on the network, so an IP address of 32 1s is considered a broadcast message to all networks and all devices. It is possible to broadcast to all machines in a network by altering the local or host address to all 1s so that the address for a Class B network is received by all devices. Coding the address as all 0s refers only to the originating device. The all-zero format is used when the network IP address is not known, but other devices on the network can still interpret the local address. By convention, no local device is given a physical address of 0. It is possible for a device to have more than one IP address if it is connected to more than one network, as is the case with gateways. This is sometimes referred to as being multihomed.

The address is reserved as the loopback address of a device. It is used for test purposes and cannot be assigned as a host ID, but here is a way to configure additional loopback addresses on a router for network-management purposes. Consider a router that has eight interfaces, all of which have a unique IP address. Remote network management systems (NMS) need a target address to reach the router in order to query its MIB. The address used is fundamentally just an open door for the NMS to collect MIB tables regarding the entire router, not just the interface associated with the address. If the circuit is down for the interface that happens to have the target address, data collection is interrupted. Most router vendors offer the capability to configure a virtual interface, using any valid IP address, as a loopback interface for network-management purposes. The main benefit is that it is available as long as the router is operational.


IP version 6 (IPv6) was designed to address the issues inherent to IPv4. The major improvement with IPv6 is the capability to handle much larger address spaces, which eliminates any threat of running out of IP addresses. In addition to scalability, IPv6 offers improved security, ease of configuration, and network management. It has been tested on a worldwide, isolated network called 6BONE, which included participants in more than 30 countries.

The major changes brought about by IPv6 are as follows:

  • Greater address space. The address space in IPv6 is 128 bits long, compared to IPv4’s 32 bits.
  • Stateless addressing. IPv6 networks can automatically route messages using the ICMPv6 discovery messages that send a broadcast to other routers with details of its network.
  • Link local address. Automatically configured in the host; valid only in the local physical link.
  • Large packet support. Enables packets up to 4GB instead of IPv4’s limit of 64KB.
  • Streamlined header that moves nonessential and optional fields to extension headers for increased efficiency in processing at intermediate nodes.

IPv6 addresses are usually written as eight groups of four hexadecimal digits separated by colons. So, if an IPv4 address is, an IPv6 address looks like 192a:0d8e:743b:92f2:a083:cf3e:6fe4:8237.

According to specifications in RFC 4292, “IPv6 Addressing Architecture,” long strings of 0s can be compressed using the special syntax ::, as long as it appears only once in an address. The double-colon syntax can also be used for leading or trailing 0s.

Figure 1-8 shows what the IPv6 header looks like.

Figure 1-8IPv6 header

The header itself is 320 bits long (40 octets) and contains the following:

  • Version. 4-bit IP version
  • Traffic class. A packet priority value
  • Flow label. Used for quality of service (QoS) management (currently unused)
  • Payload length. Number of bytes in the payload
  • Next header. Next encapsulated protocol (compatible with IPv4 values)
  • Hop length. TTL value from IPv4
  • Source address. 128-bit IPv6 address
  • Destination address. 128-bit IPv6 address

IPv6 was developed in the early 1990s. It was supposed to roll out in the late 1990s, but this never happened because of the differences in IPv4 and IPv6 and the cost of simultaneously supporting both protocols. IPv6 has been added as a viable protocol for the Internet only in the last two years, with full support along the backbone for IPv6 now in place. Although plans to phase out IPv4 in favor of IPv6 are touted, the sheer number of legacy devices that cannot support IPv6 means that a complete switchover is unlikely to happen for many years. Conversion efforts might be hastened by the U.S. Office of Management and Budget (OMB), which mandated that federal agencies convert to IPv6 by June 30, 2008. The 26 agencies in the mandate all made the deadline in some manner.


Since its official birth in 1983, the Internet has grown beyond its fashionable description as the information superhighway to a communication mechanism that is a necessity, not just a convenience. Government and commercial entities depend on its services when it is necessary to communicate with people not part of their own isolated and secure networks. The Internet is so critical to global concerns that virtually every developed country in the world now has a hand in its continued evolution. The layered set of protocols that make it work enable innovation in many forms, and technical contributions are never in short supply.

Commercial and government enterprises, those networks that are isolated from public network connectivity, mirror the Internet in many ways. Layer 2 switching technologies on the LAN connect to Layer 3 routers in a way that enables personal computers, servers, printers, and various video and voice devices to connect on a global scale.

In the midst of such extensive global communication, a constant struggle against illegal activities exists. Network security professionals must participate in every aspect of network innovation, either as inventors or as students of technology.


© Copyright Pearson Education. All rights reserved.

This story, "Chapter 1: Network Overview" was originally published by Network World.


Copyright © 2009 IDG Communications, Inc.

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