CCNA Savvy: Segmenting a Network with a Router

Segmenting a LAN with a router may not be the least expensive way to go, but it does have its benefits. You can expect to find questions on the benefits of segmenting a network with a router on the CCNA exam. There are definitely less expensive ways to segment a network, such as with a bridge, and there are certainly faster, simpler ways, such as with a switch, but a router can provide benefits these devices cannot.

Why segment a network?

Here are some of the general benefits of segmenting a LAN, regardless of how that was accomplished:

  • Keeps local traffic local: Breaking up a network into smaller segments reduces congestion on the network by reducing the overall traffic loads.
  • Increases the bandwidth available to each user: Bandwidth is a shared entity, but each segment and its users have full use of the bandwidth available. For example, if there are 100 users on a 100 Mbps segment, each user has an average of 1 Mbps of available bandwidth. If this same segment were further segmented into 10 segments with 10 users on each segment, however, then every user would have an average of 10 Mbps of available bandwidth.
  • Fewer collisions: In general, traffic tends to stay within a segment, and less traffic is routed beyond the segment to contend for access to the backbone.
  • Reduces Ethernet distance limitations: An Ethernet network has inherent distance limitations. When a network is segmented with a router (and only a router; not a bridge, and not a switch), the beginning point from which the maximum distance for the cabling is determined is re-established.

Segmenting a LAN with a router

The CCNA exam focuses on why you would segment a LAN. There are several ways to segment a LAN — with a bridge, a switch, or a router. Just in case you're curious, how you segment a LAN with a router involves some knowledge of the network, its traffic, and topology. You can just pick a point in the LAN and plug the router in, but most likely, unless you are extremely lucky, you won't see much improvement in the performance of the LAN.

Routers are used to segment fairly large networks, in terms of geography and number of nodes, or very high volume networks. In most cases, you are more likely to segment a LAN with a bridge or switch.

Here are some things to consider before you segment a LAN with a router:

  • A router can segment a LAN that includes different media types. For example, a LAN may have both Category 5 and Thinnet (coaxial) cable connecting to fiber optic cabling.
  • A router can interconnect LANs that are using different protocols, provided they are all routable.
  • A router does increase latency by adding the delay caused by the router examining each packet entirely before sending it on.
  • A router can also provide more than one active link or route to a destination. On a larger LAN, this can provide route diversity and redundancy, which are always good things.

The specific benefits of segmenting with a router

So what are the benefits of using a router to segment a LAN, and why would anyone want to do it? Excellent questions, and ones you're sure to find on the CCNA exam. There are several reasons, including that you simply have money to burn. The real reasons, the ones you should know for the exam, can be summarized as:

  • Reduced size of broadcast domains: Routers block broadcasts unless specifically instructed to forward them.
  • Smaller networks: Routers create smaller networks, as opposed to dividing a large network into smaller pieces of itself.
  • Flexible addressing: Routers segment a network by using logical, rather than physical, addresses. For example, a bridge uses the MAC (Media Access Control) or physical address to make its addressing decisions, whereas the router uses the logical or IP address.
  • Better administration: A system administrator has more management tools available when using a router, thanks to the increased memory in a router and its ability to make routing decisions based on a multitude of factors.
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