Cisco VLANS and IP Subnets

Every net architect must determine the amount of IP networks or subnets required, based on the chosen IP addressing scheme.
The architect counts the amount of subnets and records the existing and projected number of users or network devices on every net.
For every location within the network, the architect should note the following data:
1. Position and description
2. VLAN or network type
3. Number of networks and computers
The designer classifies every site by documenting the wiring closet or data center number.
VLAN or Network Type
Recording the kind of VLAN or net allows the architect to accurately guess the possible growth in the number of computers. A data VLAN might grow in size more than a VLAN that includes IP telephones. A point-to-point Layer 3 net typically does not expand further than the two original host addresses.
Number of Networks and Hosts per Network
After that, the designer calculates and lists the amount of networks and the amount of computers per network that exist in the fresh design. This calculation signifies the present address requirements. The architect can then evaluate the growth in every area to define the size of the IP network or subnet.
The wireless net requirements are identified separately. Using wirelessly connected devices raises the amount of IP addresses required without adding any new switches or ports. The network architect should select a routing protocol that meets the next requirements:
1. Classless routing operation that supports VLSM
2. Small and rare routing table updates to reduce traffic
3. Fast convergence in the event of a failure
Very often redundant and backup routes are necessary to meet the availability needs. EIGRP is usually the best choice since it can support load balancing over these extra routes. By default, EIGRP configures up to four equal cost routes to the same site in the routing table. To control the amount of routes EIGRP configures, you must use the maximum-paths command. Valid values for the maximum-paths command are from 1 to 6. If a value of 1 is used, it disables load balancing, because only 1 route can be installed in the routing table for a particular destination.
Unequal Cost Load Balancing
Sometimes, it may be mandatory to use backup links for load balancing the extraordinary traffic. Since the backup routes do not continuously have the same routing cost as the primary routes, data are not load balanced across the backup routes by default. A router in an EIGRP net can be designed to practice unequal cost load balancing by using the variance command.
A variance is a number that EIGRP uses to decide whether or not to put a particular route in the routing table. Use the variance command with a value between 1 and 128. For example you can type: Router(config-router)# variance 2
Splitting IP data in this way stops a particular path from being overloaded by heavy traffic when other paths are available.