What is NetScaler?
Simple definition: NetScaler is a hardware device (or network appliance) manufactured by Citrix, which primary role is to provide Level 4 Load Balancing. It also supports Firewall, proxy and VPN functions
Other definitions:
By Citrix: “Citrix NetScaler makes apps and cloud-based services run five times better by offloading app and database servers, accelerating app and service performance, and integrating security. Deployed in front of web and database servers, NetScaler combines high-speed load balancing and content switching, data compression, content caching, SSL acceleration, network optimization, application visibility and application security on a single, comprehensive platform.
By using NetScaler to create a services delivery fabric overlay spanning enterprise and cloud datacenters, enterprises can make the cloud a transparent extension of their own network. IT organizations can extend existing in-place processes and tooling to the cloud-based services the business adopts. ” (source: Citrix Netscaler ADC
Here are two good real examples of the benefit of NetScaler:
“Without NetScaler, we would have needed additional servers to handle growth. Avoiding buying additional servers and upgrading bandwidth saved us over 30.000 euros immediately. This is an attractive return on investment.” Ignace Quaghebeur – System Engineer, Jetair
“Our number-one project is implementing the Oracle ERP. Giving brokers and co-manufacturers secure access to the system is imperative. Citrix NetScalers was the best solution for delivering Oracle with reliability, speed and cost-effectiveness.” George Scangas – It Infastructure Manager, Welch Foods, Inc.
(Note: Both examples were extracted from the Citrix page link above)
“Netscaler optimizes application availability through advanced L4-7 load balancing and traffic management in order to accelerate performance” (Wikipedia entry)
Netscaler ADC -” A hardware and software solution providing advanced application and service delivery. Netscaler is a reverse-proxy with high-speed load balancing and content switching, data compression, content caching, SSL acceleration, network optimization, application visibility and application security on a single platform” (http://en.wikipedia.org/wiki/Reverse_proxy)
What is Level 4 Load Balancing?
The level 4 refers to the Transport layer in the OSI model. A router with NAT (Network Access Translation) feature is considered a Layer 4 router instead of a typical network router which resides in the Layer 3 of the OSI model. The layer 4 Load Balancing router uses different rules to make decisions on where to route traffic. So servers and other machines with least load or fastest response times will receive the packets from the L-4 router
Read this explanation from wikipedia for a more precise definition and better understanding:
“A typical network router simply sends incoming packets onto the appropriate IP address on its network. A layer 4 router, more correctly a NAT with port and transaction awareness, uses a little trickery and sends incoming packets to one or more machines which are hidden behind a single IP address.
The layer 4 refers to the 7 layer OSI model. The router is on the Transport Layer and makes decisions on where to send the packets. Modern load balancing routers can use different rules to make decisions on where to route traffic. This can be based on least load, or fastest response times, or simply balancing requests out. This is also a redundancy method, so if one machine is not up, the router will not send traffic to it. ” (wikipedia entry on MultiLayer_Switch )
Here are some models of the Netscaler hardware available (or discountinued) on the market
Citrix NetScaler 7000
Citrix NetScaler 9010
Citrix NetScaler 10010
Citrix NetScaler 12000
Citrix NetScaler MPX 5500
Citrix NetScaler MPX 7500 and MPX 9500
Citrix NetScaler MPX 9700, MPX 10500, MPX 12500, and MPX 15500
Citrix NetScaler MPX 17500, MPX 19500, and MPX 21500
Citrix NetScaler MPX 15000
Citrix NetScaler MPX 17000
The following link has more details about these devices (It is the Citrix NetScaler Hardware Installation and Setup Guide)
Citriland - Citrix concepts, tips, issues 
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