IP Telephony Networks  – Internet Protocol Version 4 (IPv4) Design

You should reserve separate subnets for LANs using IP phones. IP phones are normally placed in a VLAN that is in a logical segment separate from that of the user workstations. Separating voice and data on different subnets or VLANs also aids in providing QoS for voice traffic with regard to classifying, queuing, and buffering. This design rule also facilitates troubleshooting.

Table 1-23 shows an example of allocating IP addresses for a small network for a company located within three buildings. Notice that separate VLANs are used for the VoIP devices.

Table 1-23 IP Address Allocation for VoIP Networks

Overlay subnets can be used where IP subnets have already been allocated and no spare subnets are available. A different class of private address can be used. This approach solves the scalability issues with the addressing plan. Table 1-24 shows an example similar to that in Table 1-23 but using overlay subnets.

Table 1-24 Overlay IP Address Allocation for VoIP Networks

VLSM Address Assignment: Example 2

Because this topic is important, here is another example of a VLSM design, this time involving network 10.0.0.0/8, which companies commonly use in their internal networks because this is private IP address space.

Global companies divide this address space into continental regions for the Americas, Europe/Middle East, Africa, and Asia/Pacific. An example is shown in Table 1-25, where the address space has been divided into four major blocks:

• 10.0.0.0 to 10.63.0.0 is reserved.
• 10.64.0.0 to 10.127.0.0 is for the Americas.
• 10.128.0.0 to 10.191.0.0 is for Europe, Middle East, and Africa.
• 10.192.0.0 to 10.254.0.0 is for Asia Pacific.

Table 1-25 Global IP Address Allocation

From each of these regions, address blocks can be allocated to company sites. Large sites may require 4, 8, or 16 Class C equivalent (/24) subnets to assign to data, voice, wireless, and management VLANs. Table 1-26 shows an example. The large site is allocated network 10.64.16.0/20. The first four /24 subnets are assigned for data VLANs, the second four /24 subnets are assigned for voice VLANs, and the third four /24 subnets are assigned for wireless VLANs. Other subnets are used for router and switch interfaces, point-to-point links, and network management devices.

Table 1-26 IP Address Allocation in a Large Site