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Cisco IP Telephony Network Design Guide

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Chapter 2 Campus Infrastructure Considerations

IP Addressing and Management

Each IP phone requires an IP address, along with associated information such as

subnet mask, default gateway, and so on. Essentially, this means that your

organization’s need for IP addresses doubles as you assign IP phones to users.

This information can be configured statically on the IP phone, or it can be

provided by the Dynamic Host Configuration Protocol (DHCP).

The following sections describe various ways that you can meet these IP

addressing requirements:

• Assigning IP Addresses Using Same Subnet as Data Devices

• Modifying the IP Addressing Plan

• Creating a Separate IP Subnet for IP Phones

Assigning IP Addresses Using Same Subnet as Data Devices

You might want to provide IP addresses to the IP phones using the same subnet as

data devices. This might be a straightforward solution in your situation. However,

many sites have IP subnets with more than 50% of subnet addresses already

allocated. If your network fits this description, this is not the best solution for your

needs.

Modifying the IP Addressing Plan

You could assign addresses for IP phones out of the existing subnets, but you must

renumber the IP addressing plan. This may not always be feasible.

Creating a Separate IP Subnet for IP Phones

You can put the IP phones on a separate IP subnet. The new subnet could be in a

registered address space or in a private address space, such as network 10.0.0.0.

Using this scheme, the PC would be on a subnet reserved for data devices and the

phone would be on a subnet reserved for voice. Configuration on the IP phone can

be minimized by having the phone learn as much information dynamically as

possible. Therefore, when the IP phone powers up it should get its voice subnet

automatically, then send a DHCP request on that subnet for an IP address.

The automated mechanism by which the IP phone gets its voice subnet is provided

through enhancements to the Cisco Discovery Protocol (CDP).

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Cisco WS-X4604-GWY - VoIP Gateway Spezifikationen Seite 55