In a single broadcast domain, or flat network, every device is in the same network and receives each
broadcast. All hosts can be reached without going through a router.
In a hierarchical design, a network is divided into layers to reduce congestion and the size of failure
domains. A hierarchal addressing structure logically groups networks in to smaller subnetworks and can
simplify network management. It is possible to have a hierarchical network without hierarchical
addressing. Although the network still functions, the effectiveness of the network design decreases and
certain routing protocol features, such as route summarization, do not work properly.
VLSM is the concept of subnetting a subnet. Classful routing protocols, such as RIPv1, do not include a
subnet mask field with a routing update. A router with a subnet mask assigned to its interface assumes
that all packets within that same class have the same subnet mask assigned.
Benefits of VLSM:
Allows efficient use of address space
Allows the use of multiple subnet mask lengths
Breaks up an address block into smaller blocks
Allows for route summarization
Provides more flexibility in network design
Supports hierarchical enterprise networks
The use of CIDR requires a classless routing protocol, such as RIPv2 or EIGRP or static routing. To CIDRcompliant routers, address class is meaningless. The network subnet mask determines the network
portion of the address. This is also known as the network prefix, or prefix length. The class of the
address no longer determines the network address.
Classless routing protocols that can support VLSM and CIDR include interior gateway protocols (IGPs)
RIPv2, EIGRP, OSPF, and IS-IS. ISPs also use exterior gateway protocols (EGPs) such as Border Gateway
Protocol (BGP).
The difference between the classful routing protocols and classless routing protocols is that the classless
routing protocols include subnet mask information with the network address information in the routing
updates.
The sending router, by default, summarizes all of the subnets and advertises the major classful network
along with the summarized subnet mask information. This process is often referred to as summarizing
on a network boundary. While most classless routing protocols enable summarization on the network
boundary by default, the process of summarizing can be disabled.
When summarization is disabled, the sending router advertises all subnetworks with subnet mask
information.
Route summarization groups contiguous subnets or networks using a single address. Route
summarization is also known as route aggregation and occurs at a network boundary on a boundary
router.
Summarization decreases the number of entries in routing updates and lowers the number of entries in
local routing tables. It also reduces bandwidth utilization for routing updates and results in faster routing
table lookups.
To summarize routes, take the highest and lowest subnet addresses in the network and convert to
binary, find the bit match and convert to dotted decimal. That is the supernetted route.
Discontiguous networks cause unreliable or suboptimal routing. To avoid this condition, an
administrator can:
Modify the addressing scheme, if possible
Use a classless routing protocol, such as RIPv2 or OSPF
Turn automatic summarization off
Manually summarize at the classful boundary
Private address ranges
Class A: 10.0.0.0 - 10.255.255.255
Class B: 172.16.0.0 - 172.31.255.255
Class C: 192.168.0.0 - 192.168.255.255
NAT can be configured statically or dynamically. Static NAT maps a single inside local address to a single
global, or public address. Dynamic NAT uses an available pool of Internet public addresses and assigns
them to inside local addresses. The address that one internal host uses to connect to another internal
host is the inside local address. The public address assigned to the organization is called the inside global
address. The inside global address is sometimes used as the address of the external interface of the
border router.
Create the access list for the addresses that will be translated and create the pool (209.165.202.131
209.165.202.140. The ACL uses a wildcard mask which is the inverse of a subnet mask. A wildcard mask
of 0.0.0.255 is the same as a subnet mask of 255.255.255.0
PAT is the same as NAT overload. All conversations translate to the same ip address but are tagged with
a source port number.