How to Do Number Planning for Voice Intranet?

Once the extension numbers are determined and assigned to the individuals in an organization, people would stick with the numbers for years. The extension numbers can be on printed on business cards, in marketing documents, and even programmable hardware devices. When an organization grows up, e.g. more people or more sites are added to the organization, the extension numbers in use are better to be kept the same. For these reasons, the extension numbers in a multiple site organization need to be carefully planned.

IPX Code
IPX code is the identifier of an IPX system within the voice intranet of an organization. Within a voice intranet, IPX code must be unique. Different IPX systems can have IPX codes with different length. But if an IPX has multiple IPX codes, these IPX codes must have the same length. An IPX code should not be a prefix of another IPX code in the intranet.

IPX Extension
IPX extension == IPX code + suffix. The length of IPX extension must be the same across entire voice intranet. Extensions include analog extensions, IP extensions, virtual extensions, and implicit extensions that are derived from PBX inter-work setup.
For example, if the length of extension is 3 and the only IPX code on an IPX code 10, the possible extensions are 100, 101, ? 109, total 10 extensions; If the IPX has two IPX codes, 10 and 11, the possible extension will from 100 to 119, total 20 extensions; If the IPX has 1 IPX code 5, the possible extension will be from 500 to 599, total 100 extensions.
The shorter IPX code is, the more room for extensions will have. But, shorter IPX code could consume more IPX code space. For example, 8 IPX P100 use single digit IPX code 1, 2, 3,?8 respectively. When adding a new IPX to the network, no proper IPX code is available. Note 9 or 9x cannot be used as IPX code since 9 is reserved for external calls in PBX.

A network of 4 IPX P100 (say P1, ? P4), 3 IPX H500 (say H1, H2, H3), 2 IPX B2000 (say B1, B2). They are in different location. Each location has possibility of grow. The numbering plan can be:
Extension Length: 4
Extensions are: P1: 511X; P2: 521X; P3: 531X P4: 541X
    H1: 31XX; H2: 32XX; H3: 33XX
    B1: 1XXX; B2: 2XXX;


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