IP Network Design Series-IP Address Management (1)

IP address management is the foundation of a successful logical design. This section describes how to develop a Scalable IP address management plan that supports Network resizing at any time. This section also describes the use and importance of key tools such as variable length subnet masks and route aggregation. It is equally important to select an appropriate routing protocol.
The suitability parameters used to evaluate a routing protocol are also studied and discussed here. The different features of the IP routing protocol will be introduced together with the operation of industrial standard protocols such as the routing information protocol (RIP) and the Open Shortest Path Priority Protocol (OSPF.
Variable Length Subnet Mask

A Variable Length Subnet Mask (VLSM) indicates that more than one subnet mask is used in the same primary category of a network. It can more efficiently use IP address space in the host and subnet. VLSM is very important in a network with no sufficient IP address space.

To use different subnet masks on the same primary network, a routing protocol supporting VLSM is required. This routing protocol is called classless routing protocol. These protocols carry subnet mask information in route broadcasts, so they can support more than one subnet mask. Examples of classless routing protocols include OSPF, RIP version 2, Cisco's OSPF (Enhanced Internal Gateway Routing Protocol), and BGP (Border Gateway Protocol) and IS-IS (intermediate system-intermediate system protocol ).

Consider an example of using VLSM. Assume that a class B address 172.16.0.0 is required to support a network with a total of 200 sites. This busiest Lan may support a maximum of 100 hosts and can have a maximum of 400 point-to-point WAN connections. Therefore, 600 subnets are required. Each subnet can have a maximum of 100 hosts. Even if Class B addresses are used, there is not enough address space to meet this requirement without VLSM.

When planning a VLSM solution, you should first use the shortest subnet mask. In other words, you should plan to make this subnet support the most hosts. This is generally used for subnet masks in most or all LAN segments. In this example, there are 200 lan cidr blocks, each of which can support up to 100 hosts. Although seven "host bits" (the binary digits of the host address) or a 25-bit mask can meet this requirement, in terms of management, it is more convenient to use a 24-bit mask. Because VLSM is used in this example, the network address is abundant. The lan cidr block can be from 172.16.1.0/24 to 172.16.200.0/24.

Now is the second phase of VLSM. This phase includes selecting available subnets and further dividing subnets. This stage is sometimes called "dividing subnets ". It is important to remember that subnet division can be implemented only when one or more subnets are not used up.

172.16.201.0 the address range is idle and can be divided by a 30-bit mask. An extra 64 subnet is created in this address range. Similarly, you can create more than 64 subnets for point-to-point connections in the 172.16.202.x/30 address range. Each address range that can contain up to 172.16.207.x/30 can provide sufficient subnet address space for 400 serial connections. This means that the address management requirements are met, and there is still a lot of free address space.

If possible, use a continuous subnet. Although this is not important, it makes sense to select a continuous range of addresses and assign a specific subnet mask to these addresses. As will be highlighted in the next section, when discussing route aggregation, efficient IP Address allocation is not just for cleanliness, this is usually necessary for a good network design.

Route Aggregation

A route aggregation aggregates a group of routes into a single route broadcast. The final result of Route aggregation and the most obvious benefit is to reduce the size of the route table on the network. This will reduce the latency related to each route hop, because the average query time of the route table is faster because the number of Route logon entries is reduced. Due to the decrease in the number of broadcast route logon entries, the overhead of the routing protocol will also be significantly reduced. As the entire network (and the number of subnets) expands, route aggregation becomes more important.

In addition to reducing the size of the route table, route aggregation can also improve the network stability by limiting the propagation of Route communication after the network connection is disconnected. If a router sends only the aggregated route to the next downstream router, it will not broadcast changes related to the specific subnet contained in the aggregation range. For example, if a router only broadcasts the aggregation route address 172.16.0.0/16 to its neighboring router, if it detects a fault in the 172.16.10.0/24 LAN segment, it will not update the neighboring router.

This principle can significantly reduce any unnecessary route updates after the network topology changes. In fact, this will accelerate convergence and make the network more stable. A classless routing protocol is required to execute a route aggregation that can be set forcibly. However, the classless routing protocol itself is not enough. It is essential to develop an IP address management plan, so that no conflicting route aggregation can be implemented at the strategic point of the network.

These address ranges are called consecutive CIDR blocks. For example, a router that connects a group of branch offices to the Headquarters can aggregate all subnets used by these branch offices into a single route broadcast. If all these subnets are in the range of 172.16.16.0/24 to 172.16.31.0/24, the address range can be 172.16.16.0/20. This is a continuous address range consistent with bit boundary. Therefore, we can ensure that this address range can be aggregated into a single statement. To maximize the benefits of Route aggregation, it is essential to develop a detailed address management plan.

Select Routing Protocol

The importance of choosing the correct IP routing protocol has been indirectly mentioned. Now, I will introduce how to evaluate a routing protocol. Let's take a look at some of the characteristics of a route protocol.

· Stability

The routing protocol must be stable to prevent routing loop problems. A routing loop is caused by a false route information broadcast immediately after the network topology changes, which can cause the network to crash. The persistence timer (holddown timer) is used for less advanced protocols such as RIP to improve stability. If the performance of a subnet declines, all routers will ignore any updates to that subnet while the timer is running.

This routing protocol effectively adopts a "wait-and-see" method after the network topology structure changes to ensure the network stability. However, because the RIP Protocol does not maintain sufficient information to quickly and reliably converge the network, the disadvantage of using the timer is to reduce the aggregation speed. This is a last resort.