Mr BA
26-08-2009, 02:49
Only the address space for two of 512 areas is shown in Table A-1. These areas are defined with the base address 150.100.2.0. Illustrating the entire address space for 150.100.0.0 would require hundreds of additional pages of addressing information. Each area would require the equivalent number of entries for each of the example areas illustrated here.
Table A-1 illustrates the assignment of 255 IP addresses that have been split between two OSPFareas. Table A-1 also illustrates the boundaries of the subnets and of the two OSPF areas shown(area 8 and area 17).
For the purposes of this discussion, consider a network that requires point-to-point serial links in each area to be assigned a subnet mask that allows two hosts per subnet. All other subnets are to be allowed 14 hosts per subnet. The use of bit-wise subnetting and variable-length subnet masks (VLSMs) permit you to customize your address space by facilitating the division of address spaces into smaller groupings than is allowed when subnetting along octet boundaries. The address layout shown in Table A-1 illustrates a structured approach to assigning addresses that uses VLSM. Table A-1 presents two subnet masks: 255.255.255.240 and of 255.255.255.252. The first mask creates subnet address spaces that are four bits wide; the second mask creates subnet address spaces that are two bits wide.
Because of the careful assignment of addresses, each area can be summarized with a single area router configuration command (used to define address range). The first set of addresses starting with 150.100.2.0xxxxxxx (last octet represented here in binary) can be summarized into the backbone with the following command:
area 8 range 150.100.2.0 255.255.255.128
This command assigns all addresses from 150.100.2.0 to 150.100.2.127 to area 8. Similarly, the addresses from 150.100.2.128 to 150.100.2.255 for the second area can be summarized as follows:
area 17 range 150.100.2.128 255.255.255.128
This command assigns all addresses from 150.100.2.128 to 150.100.2.255 to area 17.
Allocation of subnets allows you to decide where to draw the line between the subnet and host (using a subnet mask) within each area. Note that in this example there are only seven bits remaining to use because of the creation of the artificial area mask. The nine bits to the left of the area mask are .
Table A-1 illustrates the assignment of 255 IP addresses that have been split between two OSPFareas. Table A-1 also illustrates the boundaries of the subnets and of the two OSPF areas shown(area 8 and area 17).
For the purposes of this discussion, consider a network that requires point-to-point serial links in each area to be assigned a subnet mask that allows two hosts per subnet. All other subnets are to be allowed 14 hosts per subnet. The use of bit-wise subnetting and variable-length subnet masks (VLSMs) permit you to customize your address space by facilitating the division of address spaces into smaller groupings than is allowed when subnetting along octet boundaries. The address layout shown in Table A-1 illustrates a structured approach to assigning addresses that uses VLSM. Table A-1 presents two subnet masks: 255.255.255.240 and of 255.255.255.252. The first mask creates subnet address spaces that are four bits wide; the second mask creates subnet address spaces that are two bits wide.
Because of the careful assignment of addresses, each area can be summarized with a single area router configuration command (used to define address range). The first set of addresses starting with 150.100.2.0xxxxxxx (last octet represented here in binary) can be summarized into the backbone with the following command:
area 8 range 150.100.2.0 255.255.255.128
This command assigns all addresses from 150.100.2.0 to 150.100.2.127 to area 8. Similarly, the addresses from 150.100.2.128 to 150.100.2.255 for the second area can be summarized as follows:
area 17 range 150.100.2.128 255.255.255.128
This command assigns all addresses from 150.100.2.128 to 150.100.2.255 to area 17.
Allocation of subnets allows you to decide where to draw the line between the subnet and host (using a subnet mask) within each area. Note that in this example there are only seven bits remaining to use because of the creation of the artificial area mask. The nine bits to the left of the area mask are .