Week Seven
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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 Addresses
What 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 mid2001, 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 common
10101100 . 00010000 . 00000000 . 00000000 –172.16.0.0
10101100 . 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 hostrenumbering 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
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)
Dynamic Host Configuration Protocol
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.
Classless Interdomain Routing (CIDR)
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.
Classless Interdomain Routing (CIDR)
• 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
Classless Interdomain Routing (CIDR)
Classless Interdomain Routing (CIDR)
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 Aggregation
Given 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
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
Dynamic Host Configuration Protocol
• 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-topoint 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
FLSM
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
VLSM
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 Space
Root Level Domain
Top Level Domain and Countries
(Australia com edu gov net org )
Second Level Domain
( microsoft franklin cisco )
(Seattle student)
Domain Name Space
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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.