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IP address management -- from 'Network troubleshooting and diagnostics'

Greg Shields

This tip is excerpted from "Network troubleshooting and diagnostics," Chapter 4 of The Shortcut Guide to Network Management for the Mid-Market, written by Greg Shields and published by Realtimepublishers.com. You can read the

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entire e-book for free at the link above.

The next set of tools specifically deals with the management and maintenance of IP addresses. With typical Class C subnets consuming upwards of 254 addresses per subnet and most companies needing multiple subnets, the sheer number of addresses under management can grow huge as the number of subnets increases. Getting your arms around this task can be difficult. The tools discussed in the following sections are designed to assist with that process of managing the scope of IP addresses on your network.

Subnet and address calculations

Pundits and conference speakers offer sessions on "How to Subnet in your Head in 90 Minutes." And there are whole Web sites devoted to assisting with the process of defining the hosts in a subnet based on subnet masking parameters. Thus, obviously this binary math isn't an easy process. Tools also exist that can assist with this tedious process. These tools give the administrator the ability to define subnet masks and report on the available addresses, broadcast address, and network address associated with those subnet characteristics.

Some tools provide the capability to input hosts into the resulting framework to help identify whether that subnet will provide the necessary space for the hosts in question. Good tools allow for the calculation of subnets both from the needs of residing hosts as well as by knowing the mask bit, host bit, and number of needed subnet information. When considering a subnet and address calculation tool, consider one with the following features:

  • Forward and reverse DNS lookups
  • Data export to common file formats
  • Integration with ping tools
  • Multiple calculation parameters
  • Address assignment
  • Classless Inter-Domain Routing support

DHCP

Interestingly, although the automatic assignment of addresses through the Dynamic Host Configuration Protocol (DHCP) is considered a network function, its administration is usually done by systems administrators. This is usually the case with small and medium-sized networks because the server that handles the DHCP service resides not on a network device but instead on a server.

However, the management of DHCP scopes can leak into the role of the network administrator in situations in which DHCP scopes fill up. In networks with many DHCP scopes at high utilization, when the scope fills to 100%, users interpret the resulting lack of network connectivity as a network problem. In those situations, the network administrator is often the first to be called in to troubleshoot the problem.

Including DHCP scope monitoring tools in your network administrators' toolset can help in these situations as full scope problems are difficult to track down using other tools. When considering a DHCP scope monitoring tool, look for one with the following capabilities:

  • Tabular user interface
  • Support for BIND and Windows-based DHCP
  • Alerting and notification
  • Visual identification of full and near-full scopes
Problems associated with full and nearly-full DHCP scopes can be a troubleshooting nightmare. This is because the client error messages associated with a full DHCP scope in many OSs are unclear. Also, the resolution to the problem is often a re-segmenting of the network to add new subnets. It is for this reason many networks utilize multiple full Class C networks for workstation networks.

If you are having issues with full or nearly-full scopes due to machines that repeatedly come on and off the network, consider reducing the DHCP lease time to a very short amount of time before re-segmenting the network. DHCP renewal traffic is very minimal on today's networks and the added traffic from the increased number of DHCP lease renewals should not significantly impact network performance.

IP address management tools

Where the intersection of the systems and the network administrator can cause difficulty is in the management of available IP addresses for subnets not serviced by DHCP. In typical networks, these subnets often house the network servers and server infrastructure. Because servers are critical components of the network, management of their IP address space is important to ensuring their uptime and availability.

In early networks, systems administrators often use a "ping and pray" approach to finding an available IP address on a server subnet. In this approach, they ping various addresses on the server subnet and look for the first one that does not respond. They then configure the new server with that IP address and "pray" that it wasn't in use by a server experiencing an extended outage. In dynamic situations with servers going up and down for extended periods, this can be especially problematic.

A better approach to using "ping and pray" is to incorporate an IP address management tool that monitors for use of IP addresses in critical subnets. The tool can store the last known-connected device for each IP as well as notify the administrator how long that IP address has either been used or has gone unused. When looking for such a tool, consider the following features:

  • Forward and reverse DNS lookups
  • Data export to common file formats
  • Active monitoring
  • Database storage
  • SNMP support