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{{Short description|Ability to balance computer network traffic}}
'''Network load balancing''' is the ability to balance traffic across two or more [[Wide area network|WAN]] links without using complex [[Routing protocol|routing protocols]] like [[Border Gateway Protocol|BGP]].
This capability balances network sessions like Web, <!-- "Web" should be capitalized because it is the shortened form of the proper noun "World Wide Web" --> email, etc. over multiple connections in order to spread out the amount of [[Bandwidth (computing)|bandwidth]] used by each [[Local area network|LAN]] user, thus increasing the total amount of bandwidth available. For example, a user has a single WAN connection to the [[Internet]] operating at 1.5 Mbit/s. They wish to add a second broadband (cable, DSL, wireless, etc.) connection operating at 2.5 Mbit/s. This would provide them with a total of 4 Mbit/s of bandwidth when balancing sessions.
Session balancing does just that, it balances sessions across each WAN link. When Web browsers connect to the Internet, they commonly open multiple sessions, one for the text, another for an image, another for some other image, etc. These sessions can be balanced across the available connections. An [[
Additionally, network load balancing is commonly used to provide network [[Redundancy (engineering)|redundancy]] so that in the event of a WAN link outage, access to network resources is still available via the secondary link(s). Redundancy is a key requirement for [[business continuity]] plans and generally used in conjunction with critical applications like [[VPN]]s and [[VoIP]].
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=== Multicast mode ===
Another option is to make NLB in [[Multicast routing protocol|multicast]] mode. The unicast [[IPv4]] address of the cluster is linked to a [[multicast]] MAC address. The hosts is in the cluster will never send traffic to the [[Network switch|switch]] using this MAC address with the cluster IPv4 address so one would need to create a static [[Address Resolution Protocol|ARP]] entry on the router (layer 3) in the attached network. Not all vendors will allow you to create an ARP entry where you use a unicast IP address and a multicast MAC address. Cisco publishes some examples how to set up MS NLB on Catalyst switches running [[IOS]]<ref>Cisco: [http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a0080a07203.shtml Catalyst configuration examples] {{webarchive|url=https://web.archive.org/web/20140131115541/http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a0080a07203.shtml |date=2014-01-31 }}, visited 8 June 2013</ref> and these same examples can be used for switches from many other vendors. As with NLB in unicast mode: incoming traffic towards the cluster will be flooded to all ports in the switch/VLAN and not all vendors support this setup. To limit the flooding, MS NLB now supports IGMP which should lead to the switches learning which ports are actually using the [[multicast address]],<ref>Microsoft Support knowledge base: [http://support.microsoft.com/kb/283028 KB283028: IGMP Snooping support on NLB], visited 8 June 2013</ref> but it doesn't always lead to the desired result. For example, [[Dell PowerConnect]] [[multi-layer
== Server load balancing ==
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