Week5 f2017(1)

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE I Chapter 6 1
Addressing in an
Enterprise Network

© 2006 Cisco Systems, Inc. All rights reserved. Cisco PublicITE 1 Chapter 6 2
Objectives
 Analyze the features and benefits of a hierarchical IP
addressing structure.
 Plan and implement a VLSM IP addressing scheme.
 Plan a network using classless routing and CIDR.
 Configure and verify both static and dynamic NAT.

Features & Benefits of a Hierarchical IP
Addressing Structure
 Flat networks with a single broadcast domain lose
efficiency as hosts are added
 Two solutions:
Create VLANs
Use routers in a hierarchical network design

 Classful network address in the Core Layer
 Successively smaller subnets in the Distribution and
Access Layers

Use subnetting to subdivide a network based on:
 Physical location or logical grouping
 Application and security requirements
 Broadcast containment
 Hierarchical network design

Plan / Implement a VLSM (Variable Length
Subnet Mask) Addressing Scheme
 Subnet mask: 32-bit value
 Distinguishes between network and host bits
 Can vary in length to accommodate number of hosts on
LAN segment

Plan / Implement a VLSM Addressing Scheme
 Boolean ANDing compares bits in host address to bits
in subnet mask
 1 and 1 = 1
 1 or 0 and 0 = 0
 Resulting value is network address

Steps in basic subnetting:
 Borrow bits from the host side
 Add them to the network side
 Change mask to reflect additional bits

Elements of an addressing scheme:
 Subnet number
 Network address
 Host range
 Broadcast address

Benefits of Variable Length Subnet Masks (VLSM):
 Flexibility
 Efficient use of address space
 Ability to use route summarization

 Apply masks from largest group to smallest
 Avoid assigning addresses that are already
allocated
 Allow for some growth in numbers of hosts on each
subnet

Discussion (15 min)
 Do you see CIDR
notation? Is that the
same as VLSM?
 How many subnets?
 What are the network
addresses in these
subnets? What are the
valid host addresses in
these subnets?
 Is there any problem in
the current addressing
scheme?
192.168.10.0 = Network
255.255.255.240 (/28) = Mask
Source: Chapter 5, Todd Lammle. CCNA Routing and Switching Study
Guide: Exams 100-101, 200-101, and 200-120. Sybex. 2013

A VLSM addressing scheme
 What’s
changed?
Source: Chapter 5, Todd Lammle. CCNA Routing and Switching Study
Guide: Exams 100-101, 200-101, and 200-120. Sybex. 2013

Exercise (15 min)
 Fill in the blanks with the IP addresses shown on the
right.

Solution

Plan a Network Using Classless Routing
and CIDR
Classful routing
 Default subnet masks
 Class determined by first
octet
 No subnet mask
information exchanged in
routing updates
Classless routing
 Network prefix
 Slash (/) mask
 Subnet mask information
exchanged in routing
updates

and CIDR
 Classless Inter-Domain Routing (CIDR)
 Uses address space efficiently
 Used for network address aggregation or summarizing

Route summarization (note: it was in a previous lecture):
 Use single address to represent group of contiguous subnets
 Occurs at network boundary
 Smaller routing table, faster lookups
and CIDR
How this actually works?

and CIDR
 Use routing protocols that support VLSM (e.g., RIPv2,
OSPF, EIGRP, etc.)
 Plan subnetting to complement hierarchical design
 Disable auto-summarization if necessary
 Update router IOS
 Allow for future growth

Configure and Verify Static and Dynamic
NAT
 RFC 1918: private IP address space
 Routed internally, never on the Internet
 “Hides” internal addresses from other networks

NAT
 Network Address Translation (NAT)
 NAT translates internal private addresses into one or
more public addresses
 Use on boundary routers

NAT
 Static NAT: map single inside local address to single
public address
 Dynamic NAT: use a pool of public addresses to
assign as needed

NAT
 Port Address Translation (PAT)
 Dynamically translate multiple inside local addresses to
one public address

 Set up a network, make physical connections, and
configure addressing info on hosts and routers
 Set up the static routes on Router0 and Router1
Use command like: ip route 0.0.0.0 0.0.0.0 172.16.10.2 (setting a default
route on a router)
 Set up NAT pool and rules (an access list 1 needs to be
created first)
Steps

Steps (cont’)
 Execute ping command from PC0 to 172.16.10.2
 At the same time, check the packets received on
Router1
Enable debug ip packet and you can see the info as follows
(showing the source is IP 172.16.10.1, not 192.168.10.2!) :

Summary
 Hierarchical network design groups users into subnets
 VLSM enables different masks for each subnet
 VLSM requires classless routing protocols
 CIDR network addresses are determined by prefix
length
 Route summarization, route aggregation, or
supernetting, is done on a boundary router
 NAT translates private addresses into public addresses
that route over the Internet
 PAT translates multiple local addresses into a single
public address

Week5 f2017(1)

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