VLAN is a logical group of networks created regardless of their physical location which helps to create smaller broadcast domains within a switch. For these VLANs, different ports can be assigned. Without VLANs, a switch considers all interfaces to a switch are in one broadcast domain. VLANs provide broadcast filtering, security address, summarization, traffic flow management and reduces the workload for Spanning Tree Protocol by limiting a VLAN to a single access switch. This is useful when layer 3 networks should be created in a layer 2 switch. VLANs are a kind of tagged links between switches; they can be bridged together, or a layer 3 switch or a router can inter connect them. VLANs have the same attributes like a LAN but they assign devices regardless of their physical location. Every switch has a default VLAN of VLAN 1 enabled on the switch. Although a name for a VLAN is assigned, only VLAN number is important while sending traffics. VLAN ID, which is same as the VLAN number, is added when a packet leaves a trunked port. Devices in one VLAN group use same VLAN ID. Most common VLAN protocols are dot1q and isl, and they are used for inter VLAN communication. There are two methods of assigning a VLAN; they are called static VLAN and dynamic VLAN. Static VLANs are port based and dynamic VLANs are created using softwares. VLAN standard is IEEE 802.1 Q.
What is Subnet?
Subnet or subnetwork is a sub division of an IP network. Breaking a large network in to many small networks is called subnetting. We group a network with a network mask in order to form a subnet mask. Subnetting reduces network traffic, optimize network performance and simplifies management. Subnetwork increases routing complexity, because, in a table, each subnet is represented by a separate entry. A router is needed to connect these networks. In IPv4, the main reason for subnetting is to improve efficiency and utilize limited network address. IPv4 network consists of 256 IP addresses. If only 14 IP addresses out of these 256 is assigned to a VLAN, the remaining 240 will become useless,to overcome this waste of IP addresses we can divide that network into subnets, which consists of 16 IP addresses. Then assign these addresses for the relevant group and assign other addresses for another group or save them for future use. A local network, which is a member of a global internetwork, is normally called subnet routers. An address mask is used to define a subaddress boundary, which is called a subnet mask for this particular local network.
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What is the difference between VLAN and Subnet? • VLANs allow to isolate smaller subnet on a single device. With a smaller subnet, you have fewer devices, and it makes less broadcast traffic. But this increase unicast traffic volume between networks, which makes high CPU usage. • Between VLANs and subnets one to one relationship exists, that means one subnet can be assigned to one VLAN. Although it’s possible, trying to assign more than one subnet for a VLAN is not a good network design planning. • VLAN boundary marks the end of a logical subnet. • For MPLS, creating more subnets are better than creating more VLANs, becauseMPLS creates shortcuts between IP subnets to achieve fast performance. • VLANs are useful when we need to create IPsubnets that spreads in a wide area like university when connecting faculties or buildings. • Simply, VLAN = a broadcast domain = IP subnet.
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