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CLI-based Configuration Guide - IP Service

AR500, AR510, AR531, AR550, AR1500, and AR2500 V200R010

This document describes the concepts and configuration procedures of IP Service features on the device, and provides the configuration examples.
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Subnetting

Subnetting

A network can be divided into multiple subnets to conserve IP address space and support flexible IP addressing.

When many hosts are distributed on an internal network, the internal host IDs can be divided into multiple subnet IDs to facilitate management. Then the entire network contains multiple small networks.

Subnetting is implemented within the internal network. The internal network has only one network ID for the external network. When packets are transmitted from the external network to the internal network, the device on the internal network selects a route for the packets based on the subnet ID and finds the destination host.

Figure 1-4 shows subnetting of a Class B IP address. The subnet mask consists of a string of continuous 1s and 0s. 1s indicate the network ID and the subnet ID field, and 0s indicate the host ID.

Figure 1-4  Subnetting of IP addresses

As shown in Figure 1-4, the first 5 bits of the host ID is used as the subnet ID. The subnet ID ranges from 00000 to 11111, allowing a maximum of 32 (25) subnets. Each subnet ID has a subnet mask. For example, the subnet mask of the subnet ID 11111 is 255.255.248.0. After performing an AND operation on the IP address and the subnet mask, you can obtain the network address.

Subnetting reduces the available IP addresses. For example, a Class B IP address contains 65534 ((216 − 2)) host IDs. After 5 bits in the host ID are used as the subnet ID, there can be a maximum of 32 subnets, each having an 11-bit host ID. Each subnet has a maximum of 2046 host IDs (211 - 2, excluding the host IDs with all 1s and all 0s). Therefore, the IP address has a maximum of 65472 (32 x 2046) host IDs, 62 less than the maximum number of host IDs before subnetting.

To implement efficient network planning, subnetting and IP addressing should abide by the following rules.

Hierarchy

To divide a network into multiple layers, you need to consider geographic and service factors. Use a top-down subnetting mode to facilitate network management and simplify routing tables. In most cases:

  • A network consisting of a backbone network and a MAN is divided into hierarchical subnets.
  • An administrative network is divided into subnets based on administrative levels.

Consecutiveness

Consecutive addresses facilitate route summarization on a hierarchical network, which greatly reduces the number of routing entries and improves route search efficiency.

  • Allocate consecutive IP addresses to each area.
  • Allocate consecutive IP addresses to devices that have the same services and functions.

Scalability

When allocating addresses, reserve certain addresses on each layer to ensure consecutive address allocation in future network expansion.

A backbone network must have enough consecutive addresses for independent autonomous systems (ASs) and further network expansion.

Efficiency

When planning subnets, fully utilize address resources to ensure that the subnets are sufficient for hosts.

  • Allocate IP addresses by using variable-length subnet masking (VLSM) to fully use address resources.
  • Consider the routing mechanisms in IP address planning to improve address utilization efficiency in the allocated address spaces.
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Updated: 2019-05-20

Document ID: EDOC1100034231

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