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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Layer 3-Based VLANs
Layer 3-based VLANs use network-layer addresses (subnet address for TCP/IP) to determine VLAN
membership. These VLANs are based on layer 3 information, but this does not constitute a ‘routing’
function.
The DES-3326 allows an IP subnet to be configured for each 802.1Q VLAN that exists on the switch.
Even though a switch inspects a packet’s IP address to determine VLAN membership, no route
calculation is performed, the RIP protocol is not employed, and packets traversing the switch are
bridged using the Spanning Tree algorithm.
A switch that implements layer 3 (or ‘subnet’) VLANs without performing any routing function between
these VLANs is referred to as performing ‘IP Switching’.
IP Addressing and Subnetting
This section gives basic information needed to configure your Layer 3 switch for IP routing. The
information includes how IP addresses are broken down and how subnetting works. You will learn how
to assign each interface on the router an IP address with a unique subnet.
Definitions
• IP Address – the unique number ID assigned to each host or interface on a network. IP
addresses have the form xxx.xxx.xxx.xxx.
• Subnet – a portion of a network sharing a particular network address.
• Subnet mask – a 32-bit number used to describe which portion of a Network Address refers to
the subnet and which portion refers to the host. Subnet masks have the form xxx.xxx.xxx.xxx.
• Interface – a network connection
• IP Interface – another name for subnet.
• Network Address – the resulting 32-bit number from a bitwise logical AND operation performed
between an IP address and a subnet mask.
• Subnet Address – another name for network address.
IP Addresses
The Internet Protocol (IP) was designed for routing data between network sites. Later, it was adapted for
routing between networks (referred to as “subnets”) within a site. The IP defines a way of generating a
unique number that can be assigned each network in the internet and each of the computers on each
of those networks. This number is called the IP address.
IP addresses use a “dotted decimal” notation. Here are some examples of IP addresses written in this
format:
1. 210.202.204.205
2. 189.21.241.56
3. 125.87.0.1
This allows IP address to be written in a string of 4 decimal (base 10) numbers. Computers can only
understand binary (base 2) numbers, and these binary numbers are usually grouped together in bytes,
or eight bits. (A bit is a binary digit – either a “1” or a “0”). The dots (periods) simply make the IP
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