4: Addressing in an Enterprise Network

© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1Version 4.0

4: Addressing in an Enterprise Network

Introducing Routing and Switching in the Enterprise

© 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 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.


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


Flat and Hierarchical Networks

Using routers is another solution.


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.


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.


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


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


Subnet Mask

The subnet mask is a 32-bit value used with the IPv4used with the IPv4 address that specifies the network portion of the network portion of the address to the network devicesaddress to the network devices, ie it uses 1s1s and 0s0s to indicate which bits of the IPv4 address are network network bitsbits and which are host bits.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 maskANDed with its subnet mask to determine the network address.


Basic Subnetting ProcessInformation 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.


ACTIVITY 4.2.1


ACTIVITY 4.2.2.3


ACTIVITY 4.2.2.3


Basic Subnetting


VLSMBasic 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


VLSM


VLSM


Activity 4.2.4.3


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


• 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.


Implementing a VLSM Addressing Scheme










Activity 4.2.5.4


Classful and Classless Routing Classful routing Default subnet masks

Class determined by first octet

No subnet mask information exchanged in routing updates

Classless routing 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.

Subnet mask information exchanged in routing updates


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.


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


CIDR and Route Summarization

Use single address to represent group of contiguous subnets

Occurs at network boundary

Smaller routing tables, faster lookups


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.


CIDR and Summarization – Activity 4.3.2.3


CIDR and Summarization – Activity 4.3.2.3


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!


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


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


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?


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


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.


Examples


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


Question


Answer


Question


Answer


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

Documents

4: Addressing in an Enterprise Network