Download The Internet and Its Uses

Addressing in an
Enterprise Network
Introducing Routing and Switching in the Enterprise –
Chapter 4
Version 4.0
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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.
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Hierarchical  Organized
 Imagine a mechanic’s tools.
 Envision these tools thrown into a big box. How much
longer will it take to find a specific sized screwdriver or
wrench in this mess? Much longer…
 A flat network is like a messy toolbox. One large
broadcast domain means that every device in the
network receives each broadcast. Traffic delays and
timeouts occur, which may slow the network to a crawl.
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A Flat IP Addressing Structure
 In a single broadcast domain, or flat network, every
device is in the same network and receives each
broadcast. In small networks, a single broadcast
domain is acceptable.
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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
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A Flat IP Addressing Structure
 As your network locations grow, you segment with
routers.
Eventually, this growth becomes harder to organize
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Features & Benefits of a Hierarchical IP
Addressing Structure
 Classful network address in the Core Layer
 Successively smaller subnets in the Distribution and
Access Layers
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Features & Benefits of a Hierarchical IP
Addressing Structure
Use subnetting to subdivide a network based on:
 Physical location or logical grouping
 Application and security requirements
 Broadcast containment
 Hierarchical network design
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Plan / Implement a VLSM 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
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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
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Activity – Are these the same network?
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Plan / Implement a VLSM Addressing Scheme
Steps in basic subnetting:
 Borrow bits from the host side
 Add them to the network side
 Change mask to reflect additional bits
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Plan / Implement a VLSM Addressing Scheme
Elements of an addressing scheme:
 Subnet number
 Network address
 Host range
 Broadcast address
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Plan / Implement a VLSM Addressing Scheme
Benefits of Variable Length Subnet Masks (VLSM):
 Flexibility
 Efficient use of address space
 Ability to use route summarization
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Plan / Implement a VLSM Addressing Scheme
 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
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VLSM Example
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Activity - How many hosts?
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Now Do Another – Pick the Addresses
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Plan a Network Using Classless Routing
and CIDR
Classful routing
Classless routing
 Default subnet masks
 Network prefix
 Class determined by first
octet
 Slash (/) mask
 No subnet mask
information exchanged in
routing updates
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 Subnet mask information
exchanged in routing
updates
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Plan a Network Using Classless Routing
and CIDR
 Classless Inter-Domain Routing (CIDR)
 Uses address space efficiently
 Used for network address aggregation or summarizing
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Plan a Network Using Classless Routing
and CIDR
Route summarization:
 Use single address to represent group of contiguous
subnets
 Occurs at network boundary
 Smaller routing table, faster lookups
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Plan a Network Using Classless Routing
and CIDR
 Discontiguous subnets cause unreliable routing
 Avoid separating subnets with a different network
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Activity – Route Summarization
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Plan a Network Using Classless Routing
and CIDR
 Use routing protocols that support VLSM
 Plan subnetting to complement hierarchical design
 Disable auto-summarization if necessary
 Update router IOS
 Allow for future growth
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Calculating Route Summarization
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Discontiguous Networks
Classful routing results in each router
advertising the major Class C network
without a subnet mask.
As a result, the middle router receives
advertisements about the same network
from two different directions.
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Private IP Space and NAT
 Using private addressing has these benefits:
It alleviates the high cost associated with the purchase of public
addresses for each host.
It allows thousands of internal employees to use a few public
addresses.
It provides a level of security, because users from other
networks or organizations cannot see the internal addresses.
 RFC 1918 governs the use of the private address
spacing.
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
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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
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Configure and Verify Static and Dynamic
NAT
 Network Address Translation (NAT)
 NAT translates internal private addresses into one or
more public addresses
 Use on boundary routers
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Configure and Verify Static and Dynamic
NAT
 Static NAT: map single inside local address to single
public address
 Dynamic NAT: use a pool of public addresses to
assign as needed
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Configure and Verify Static and Dynamic
NAT
 Port Address Translation (PAT)
 Dynamically translate multiple inside local addresses to
one public address
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Static NAT example
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Dynamic NAT example
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Using PAT
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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
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