Unit 7 Subnetting Lab 7
Read the lab details listed below, and then answer the questions listed in the lab exercise.
CIDR (Classless InterDomain Routing) and VLSM
CIDR (Classless Inter-Domain Routing) was introduced in 1993 (RCF 1517) replacing the previous generation of IP address syntax - classful networks. CIDR allowed for more efficient use of IPv4 address space and prefix aggregation, known as route summarization or supernetting.
CIDR introduction allowed for:
• More efficient use of IPv4 address space
• Prefix aggregation, which reduced the size of routing tables
CIDR allows routers to group routes together to reduce the bulk of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity. With CIDR, IP addresses and their subnet masks are written as four octets, separated by periods, followed by a forward slash and a two-digit number that represents the subnet mask e.g.
CIDR / VLSM Network Addressing Topology Example CIDR uses VLSM (Variable Lenght Subnet Masks) to allocate IP addresses to subnetworks according to need rather than class. VLSM allows for subnets to be further divided or subnetted into even smaller subnets. Simply, VLSM is subnetting a subnet.
With CIDR, address classes (A, B, and C) became meaningless. The network address are no longer determined by the value of the first octet, but assigned prefix length (subnet mask) address space. The number of hosts on a network, could now be assigned a specific prefix depending upon the number of hosts needed for that network.
Propagating CIDR supernets or VLSM subnets require a classless routing protocols. A classless routing protocol includes the subnet mask along with the network address in the routing update.
Summary Routes Determination
Determining the summary route and subnet mask for a group of networks can be done in three