1. Addressing in an
Enterprise Network
Introducing Routing and Switching in the Enterprise –
Chapter 4
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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.
© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2

3. 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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4. 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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5. 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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6. 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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7. 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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8. 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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9. Plan / Implement a VLSM Addressing Scheme
Elements of an addressing scheme:
 Subnet number
 Network address
 Host range
 Broadcast address
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10. 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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11. 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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12. Plan a Network Using Classless Routing
and CIDR
Classful routing Classless routing
 Default subnet masks  Network prefix
 Class determined by first  Slash (/) mask
octet
 Subnet mask information
 No subnet mask exchanged in routing
information exchanged in updates
routing updates
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13. 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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14. 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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15. Plan a Network Using Classless Routing
and CIDR
 Discontiguous subnets cause unreliable routing
 Avoid separating subnets with a different network
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16. 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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17. 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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18. 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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19. 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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20. 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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21. 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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22. © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 22