Download Chapter 4: Addressing

4: Addressing in an
Enterprise Network
Introducing Routing and Switching in the Enterprise
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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Flat and Hierarchical Networks
 Flat networks with a single broadcast domain lose
efficiency as hosts are added
 ONE Solution! Create VLANs, each VLAN = a subnet
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Flat and Hierarchical Networks
 Using routers is another solution.
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Hierarchical IP Addressing Structure
 An effective hierarchical address scheme consists of a classful network
address in the Core Layer that is subdivided into successively smaller
subnets in the Distribution and Access Layers.
 The following is a non-hierarchical networking scheme.
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Hierarchical IP Addressing Structure
 A hierarchical addressing structure logically groups networks into
smaller subnetworks.
 An effective hierarchical address scheme consists of a classful network
address in the Core Layer that is subdivided into successively
smaller subnets in the Distribution and Access Layers.
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Using Subnetting to Structure the Network
Some reasons for subnetting are:
 Physical location (eg remote offices) or logical grouping
 Application requirements
 Security
 Broadcast containment
 Hierarchical network design
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Using Subnetting to Structure the Network
For example:
• 10.0.0.0 network for the enterprise
• Use an addressing scheme such as 10.X.Y.0
• X represents a geographical location
• Y represents a building or floor within that location
This addressing scheme allows for:
• 255 different geographical locations
• 255 buildings in each location
• 254 hosts within each building
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Subnet Mask
 The subnet mask is a 32-bit value used with the IPv4
address that specifies the network portion of the
address to the network devices, ie it uses 1s and 0s
to indicate which bits of the IPv4 address are network
bits and which are host bits.
 A /24 prefix represents a subnet mask of 255.255.255.0
(11111111.11111111.11111111.00000000).
The first 3 octets are all 1s, the remaining bits are 0s.
 Inside the network device, the IPv4 host address is
logically ANDed with its subnet mask to determine
the network address.
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Basic Subnetting Process
Information can be determined by looking at only an IP address and slash notation (/x) subnet
mask, eg an IP address of 192.168.1.75 /26 :Decimal subnet mask
The /26 translates to a subnet mask of 255.255.255.192
Number of subnets created
Assuming we started with the default /24 subnet mask, we borrowed 2 additional
host bits for the network. This creates 4 subnets (22 = 4)
Number of usable hosts per subnet
Six bits are left on the host side creating 62 hosts per subnet (26 = 64 - 2 = 62)
Network address
Using the subnet mask to determine the placement of network bits, the value of the
network address is given. In this example 192.168.1.64 (256 – 192 = 64)
First usable host address
A host cannot have all 0s within the host bits, because that represents the
network address of the subnet.
Therefore, the first usable host address within the .64 subnet is .65
Broadcast address
A host cannot have all 1s within the host bits because that represents the
broadcast address of the subnet. In this cast, the broadcast address is .127
192.168.1.128 is the network address of the next subnet.
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ACTIVITY 4.2.1
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ACTIVITY 4.2.2.3
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ACTIVITY 4.2.2.3
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Basic Subnetting
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VLSM
Basic subnetting is sufficient for smaller networks
Does not provide the flexibility needed in larger enterprise networks.
Benefits of Variable Length Subnet Masks (VLSM) are:
 Flexibility
 Efficient use of address space
 Ability to use route summarization
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VLSM
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VLSM
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Activity 4.2.4.3
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Implementing 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
 Use tools such as Charts, etc
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• Subnet of /26 is required to accommodate the largest network segment
of 58 hosts.
Using a basic subnetting scheme is not only wasteful, but creates only
four subnets.
This is not enough to address each of the required seven LAN/WAN
segments.
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Implementing a VLSM Addressing Scheme
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Implementing a VLSM Addressing Scheme
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Implementing a VLSM Addressing Scheme
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Implementing a VLSM Addressing Scheme
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Implementing a VLSM Addressing Scheme
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Activity 4.2.5.4
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Classful and Classless Routing
Classful routing
Classless routing
 Default subnet masks
 Network subnet mask
determines the network portion
of the address.
Known as the network prefix,
or prefix length.
Class of the address no longer
determines the network
address.
 Class determined by first octet
 No subnet mask information
exchanged in routing
updates
 Subnet mask information
exchanged in routing updates
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Processing RIPv1 Updates
Rule 1: if a routing update and the interface
on which it is received belong to the same
major network, the subnet mask of the
interface is applied to the network in the
routing update.
Rule 2: if a routing update and the interface
on which it is received belong to different
major networks, the classful subnet mask
of the network is applied to the network in the
routing update.
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Classless Routing and CIDR
 Classless Inter-Domain Routing (CIDR) uses address
space more efficiently
 Used for network address aggregation or summarizing
(reducing the size of routing tables)
 CIDR requires a classless routing protocol, such as
RIPv2 or EIGRP
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CIDR and Route Summarization
 Use single address to represent group of contiguous
subnets
 Occurs at network boundary
 Smaller routing tables, faster lookups
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Example
 A border router advertises all of the networks within an enterprise to the ISP.
 If there are 8 different networks, the router would have to advertise all 8.
If every enterprise followed this pattern, the routing table of the ISP would be
huge.
 Using route summarization, a router groups the networks together and
advertises them as one large group.
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CIDR and Summarization – Activity 4.3.2.3
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CIDR and Summarization – Activity 4.3.2.3
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Calculating Route Summarization
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Calculating Route Summarization
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Calculating Route Summarization
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Calculating Route Summarization
If a contiguous hierarchical addressing scheme is
not used, it may not be possible to summarize
routes. If the network addresses do not have
common bits from left to right, a summary mask
cannot be applied.
BEWARE! Do not advertise addresses that do
not belong to you!
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Example of Discontinuous Subnets
• 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.
• 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
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Subnetting Best Practices
 Use routing protocols that support VLSM
 Disable auto-summarization if necessary
 Ensure router IOS supports subnet zero
 Use /30 ranges for WAN links (ie P2P links)
 Allow for future growth
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Private Addresses and NAT
 RFC 1918 - private IP address space, available for anyone to use on
their internal network
 Routed internally, never on the Internet
 Class A: 10.0.0.0 - 10.255.255.255 /8
 Class B: 172.16.0.0 - 172.31.255.255 /12
 Class C: 192.168.0.0 - 192.168.255.255 /16
Q: What is the netmask for the
172.16.x.x network shown above?
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NAT
 Network Address Translation (NAT) translates internal private
addresses into one or more public addresses for routing onto the
Internet.
 NAT changes the private IP source address inside each packet to
a publicly registered IP address before sending it out onto the
Internet.
 Use on boundary routers
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Static and Dynamic NAT
 Static NAT maps a single inside local address to a
single global, or public address. This mapping ensures
that a particular inside local address always associates
with the same public address.
Static NAT ensures that outside devices consistently
reach an internal device such as a web server.
 Dynamic NAT uses an available pool of Internet public
addresses and assigns them to inside local addresses.
Dynamic NAT assigns the first available IP address in
the pool of public addresses to an inside device.
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Examples
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PAT
 Port Address Translation (PAT) is a variation on dynamic NAT –
sometimes known as NAT Overload
 When a source host sends a message to a destination host, it uses a
combination of an IP address and a port number (above 1024) to
keep track of each individual conversation.
10.0.0.3
10.0.0.3: 1444
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Question
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Answer
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Question
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Answer
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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 ie one-to-one, one-to-many
 PAT translates multiple local addresses into a single
public address ie many-to-one
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