week seven attendance announcements current week information upcoming assignments review multiple...

Week Seven• Attendance• Announcements• Current Week Information• Upcoming Assignments• Review multiple question midterm exam

Week Seven Topics

• Private, public, and NAT addressing• Static or Dynamic IP Address Assignment• Hierarchical Addressing, route summarization,

CIDR• Domain Name Server (DNS)

Private AddressesWhat has happened to IPv4 addresses?

In 1981, IPv4 Protocol was published. In 1985, about 1/16 of the total IPv4 address space was in use. By mid-2001, about 2/3 of the total IPv4 address space was in use.

Since 2001, Internet service providers have been trying to conserve IP addresses. They have assigned only a subset of addresses to customers. Currently, the number of public IP addresses available is insufficient for assigning addresses for an entire network.

The answer to this problem is to assign private addresses within a network and to translate these private addresses to public addresses when Internet connectivity is required.

IP Address Design Strategy

Are there public, private, or both types of addressing required?

How many end systems will need access to the public network? This includes email, file transfer, or web browsing.

How many end systems require access to visible public network(s). This includes e-commerce, such as web servers, database servers, application servers, and public servers. These end systems require globally unambiguous IP addresses.

Where will the boundaries be between private and public IP addresses and how will they be implemented?

Private Addresses• RFC 1918 sets aside three blocks of private IP

addresses:• One (1) Class A address• Sixteen (16) Class B addresses• Two hundred and fifty six (256) Class C addresses • These addresses are for private, internal network use

only • Packets containing these addresses are not routed

over the Internet • A router should never route RFC 1918 addresses,

because ISPs typically configure the border routers to prevent privately addressed traffic from being forwarded

Private Addressing

172.16.0.0 –172.31.255.255: 172.16.0.0/12

Where does the /12 come from?12 bits in common10101100 . 00010000 . 00000000 . 00000000 –172.16.0.010101100 . 00011111 . 11111111 . 11111111 –172.31.255.255-------------------------------------------------------------10101100 . 0001000 00000000 . 00000000 –172.16.0.0/12

Network Address Translation (NAT)• NAT is defined by RFC 1631. It is the process of

swapping one address for another in the IP packet header.

• NAT is a mechanism for conserving registered IP addresses in large networks and simplifying IP addressing management tasks.

• In practice, NAT is used to allow hosts that are privately addressed, using RFC 1918 addresses, to access the Internet

• NAT allows many hosts on an inside network to communicate on the Internet with one valid, assigned IP address

Network Address Translation (NAT)• NAT provides a level of security for your inside

network from the outside world

NAT Terminology• Inside local IP address: The IP address assigned to a

host on the inside network. The address is typically an RFC 1918 address.

• Inside global IP address: A globally unique IP address (typically assigned by an ISP) that represents one or more inside local IP addresses to the outside world.

• Outside global IP address: The IP address assigned to a host on the outside network by its owner. The address is globally unique.

• Outside local IP address: The local IP address assigned to a host on the outside network. In most situations, this address will be identical to the outside global address of that outside device

NAT Terminology

Static IP Address Assignment• An IP address is manually assigned to a device or

host• The network administrator configures the IP address,

default gateway, and name servers manually by entering them into a special file or files on the end system with either a graphical or text interface

• Static address assignment is an extra burden for the administrator—especially on large-scale networks—who must configure the address on every end system in the network

• Typically, routers, switches, servers, and printers have IP addresses statically assigned

Dynamic IP Address Assignment • IP addresses are automatically assigned to the devices• Dynamic address assignment relieves the administrator of

manually assigning an address to every network device• Instead, the administrator must set up a server to assign the

addresses. • On that server, the administrator defines the address pools and

additional parameters that should be sent to the host (default gateway, name servers, time servers, and so forth)

• On the host, the administrator enables the host to acquire the address dynamically; this is often the default.

• When IP address reconfiguration is needed, the administrator reconfigures the server, which then performs the host-renumbering task

• DHCP is the protocol used to distribute these IP addresses

Dynamic Host Configuration Protocol (DHCP)

DHCP is a superset of the BootP protocol. This means that it uses the same protocol structure as BootP, but it has enhancements added. Both of these protocols use servers that dynamically configure clients when requested. The two major enhancements are address pools and lease time.

Dynamic Host Configuration Protocol

Dynamic Host Configuration Protocol (DHCP)

• A DHCP Server can provide the following to a client:

• IP address• Gateway address• Subnet mask• DNS server address• Subnet mask• Router• Domain Name• Domain Name Server(s)• WINS Server(s)

DHCP Operation

• A client must have DHCP configured when starting the network membership process

• The client sends a request to a server requesting an IP configuration

• Sometimes the client may suggest the IP address it wants, such as when requesting an extension to a DHCP lease

• The client locates a DHCP server by sending a broadcast called a DHCPDISCOVER

IP Address Assignments in an Enterprise Network

Classless Interdomain Routing (CIDR)

• CIDR is the abbreviation for “Classless InterdomainRouting”

• CIDR is pronounced “cider”• CIDR replaced the old process of assigning

addresses based on Class A, Class B, and Class C.


A method supported by classless routing protocols, such as OSPF and BGP4, based on the concept of ignoring the IP class of address, permitting route aggregation and VLSM that enable routers to combine routes in order to minimize the routing information that needs to be conveyed by the primary routers. It allows a group of IP networks to appear to other networks as a unified, larger entity. In CIDR, IP addresses and their subnet masks are written as four dotted octets, followed by a forward slash and the numbering of masking bits.


• With CIDR, addresses use bit identifiers, or bit masks, instead of an address class to determine the network portion of an address

• CIDR uses the /N notation instead of subnet masks

• CIDR allows for the more efficient allocation of IP addresses

• Blocks of addresses that match an organization’s needs can be issued


172.16.0.0 255.255.0.0 = 172.16.0.0 /16

198.30.1.0 255.255.255.0 = 198.30.1.0 /24

Note that 192.168.24.0 /22 is not a Class C network, it has a subnet mask of 255.255.252.0

CIDR and Route Aggregation• CIDR allows routers to summarize, or

aggregate, routing information• One address with mask can represent multiple

networks• This reduces the size of routing tables• Supernetting is another term for route

aggregation

CIDR and Route AggregationGiven four Class C Networks (/24):

192.168.16.0 11000000 10101000 00010000 00000000

192.168.17.0 11000000 10101000 00010001 00000000

192.168.18.0 11000000 10101000 00010010 00000000

192.168.19.0 11000000 10101000 00010011 00000000

Identify which bits all these networks have in common. 192.168.16.0 /22 can represent all these networks. The router will look at the first 22 bits of the address to make a routing decision

CIDR and Route Aggregation

Route Summarization

Importance of Hierarchical Addressing

Without summarization, every small change in the network will be propagated (spread)

throughout the entire network

Importance of Hierarchical Addressing

With summarization, small changes in the network aren’t propagated (spread) throughout the entire network

Benefits of Summarization

Subnet Masks

• A major network is a Class A, B, or C network• Fixed-Length Subnet Masking (FLSM) is

when all subnet masks in a major network must be the same

• Variable-Length Subnet Masking (VLSM) is when subnet masks within a major network can be different. In modern networks, VLSM should be used to conserve the IP addresses

• Some routing protocols require FLSM; others allow VLSM


• FLSM requires that all subnets of a major network have the same subnet mask, which therefore results in less efficient address space allocation.

• The network on the next slide is composed of multiple LANs that are connected by point-to-point WAN links.

• Because FLSM is used, all subnets have the same subnet mask. This is inefficient, because even though only two addresses are needed on the point-to-point links, a /24 subnet mask with 254 available host addresses is used

VLSM

• VLSM makes it possible to subnet with different subnet masks and therefore results in more efficient address space allocation.

• VLSM also provides a greater capability to perform route summarization, because it allows more hierarchical levels within an addressing plan.

• VLSM requires prefix length information to be explicitly sent with each address advertised in a routing update

Classful and Classless Routing Protocols• Classful routing protocols DO NOT send subnet

mask information in their routing updates• When a router receives a routing update, it

simply assumes the default subnet mask (Class A, B, or C)

• VLSM cannot be used in networks that use Classful routing protocols

• Classless routing protocols send the subnet mask (prefix length) in their updates

• VLSM can be used with Classless routing protocols

Classful versus Classles

Classful Versus Classless• When subnet masks aren’t sent in updates, routing problems

can occur• This particular problem occurs because the two 172.16.x.x

networks are separated by another network. The two networks are discontinuous

• The network is not hierarchical and appears to be a poor network design, but this may have occurred because two different networks were joined together at a later time

Classful and Classless Routing Protocols

• Classful protocols use address classes (A,B,C) to determine networks because subnet masks are not sent in routing updates

Features of Classless Routing Protocols

• The routing updates include subnet masks.• VLSM is supported.• Automatic route summarization at the major

network boundary is not required, and route summarization can be manually configured.

• Subnetted networks can be discontinuous

Domain Name Server (DNS)

Name Resolution with DNS

DNS Components• Resolver – The DNS client that sends queries

to a Name Server• Name Servers –The DNS component that

responds to queries and has the name to IP address mappings

• Domain Name Space –The hierarchical system of names used on the Internet

Domain Name SpaceRoot Level Domain

Top Level Domain and Countries(Australia com edu gov net org )

Second Level Domain( microsoft franklin cisco )

(Seattle student)

Domain Name Space• At the top is named root or .• TLD is a top level domain• The next layer is the second level domain• A second level domain may have sub-domains• Then you have host names which completely

identify a host with the FQDN (Fully Qualified Domain Name)

Upcoming Deadlines• Assignment 6-1, Concept Questions 5 due

October 20, 2010.• Assignment 8-1, Midterm exam • Assignment 1-4-2 Network Design Project

Phase 2: WAN Network Design due November 10, 2010

• Assignement 8-2 Concept Questions 6 due November 3, 2010.

week seven attendance announcements current week information upcoming assignments review multiple...

Documents

public addresses

ip address slide

registered ip addresses

unambiguous ip addresses

ipv4 addresses

private addresses rfc

blocks of private ip

subset of addresses