It is difficult and expensive to accurately determine the load being placed on a network, but a reasonable and useful estimate can be obtained by examining the number of "collisions" occurring on the network interface. By carrying out this exercise on a number of systems distributed around the network, a more accurate estimate can be calculated. In addition, this helps to identify network segments of particularly high (or low) usage. A simple and effective way of gathering this information is to run the netstat command with the -i switch. Listing 13.1 contains some sample output. Listing 13.1 Sample Output from Using the Command netstat -i to Gather Network Interface Statistical Informationtaurus# netstat -i 3 input le0 output input (Total) output packets errs packets errs colls packets errs packets errs colls 221981 5 302743 1 29087 283625 5 364387 1 29087 981 0 2095 0 670 991 0 2105 0 670 945 0 2025 0 847 953 0 2033 0 847 880 0 1901 0 1416 890 0 1911 0 1416 862 0 1862 0 1306 868 0 1868 0 1306 596 0 1249 0 857 606 0 1259 0 857 864 0 1872 0 417 872 0 1880 0 417 963 0 2083 0 588 971 0 2091 0 588 955 0 2050 0 535 963 0 2058 0 535 997 0 2114 0 532 1007 0 2124 0 532 928 0 2002 0 538 936 0 2010 0 538 1012 0 1980 0 505 1020 0 1988 0 505 947 0 2071 0 518 955 0 2079 0 518 935 0 2016 0 551 945 0 2026 0 551 978 0 2112 0 583 986 0 2120 0 583 934 0 2012 0 519 944 0 2022 0 519 986 0 2121 0 570 994 0 2129 0 570 923 0 2008 0 508 933 0 2018 0 508 944 0 2052 0 484 952 0 2060 0 484 975 0 2105 0 568 985 0 2115 0 568 432 0 986 0 295 440 0 994 0 295 350 0 824 0 235 358 0 832 0 235 360 0 850 0 251 368 0 858 0 251 371 0 877 0 231 381 0 887 0 231 360 0 853 0 247 368 0 861 0 247 373 0 879 0 260 383 0 889 0 260 taurus# The sample output from Listing 13.1 is useful because it shows a heavily loaded network interface. The line in bold text displays a set of typical values that can be used to see the current collision rate at a glance. The interface counted 2,095 output packets ”that is, packets of data that were sent onto the network. Of those packets, 670 resulted in a collision ”meaning that they never arrived and had to be retransmitted. To calculate an approximate collision rate (as a percentage), divide the number of collisions by the number of output packets and multiply by 100: Collision rate = (670 · 2095) x 100 = 31.9% In this example, 31% of packets being transmitted onto the network are colliding with other packets and require retransmission. A network functioning normally should expect the collision rate to be 2% or less. A collision rate of between 2% and about 8 “10% depicts a normal, busy network, but anything over 10% is a clear indication that the network (or at least the segment relating to the system being examined) is too heavily loaded and needs to be addressed. The previous example describes a nonswitched Ethernet network, where collisions are often an issue to be addressed. Of course a Token Ring network, including FDDI, does not suffer in the same way that an Ethernet network does because the use of a token determines which (single) host can send traffic. In a switched network, the "switching" of traffic virtually eliminates the issue of collisions and is a solution to a heavy-loaded nonswitched Ethernet network, as is subnetting. The next section describes subnetting, a method of segmenting the network to reduce the circulation of network traffic and thus reduce the collision rate. It also discusses some other benefits that subnetting provides. SubnetsSubnetting involves segmenting an existing network into a number of smaller, separate networks. Subnetting might be considered to be a valid option for several reasons:
Benefits of SubnetsSplitting a network into several smaller subnetworks has a number of advantages:
Of course, subnetting is not just a case of deciding what to do and then changing the network mask. Three basic steps are involved:
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