IP Addressing

1
IP Addressing

• Semester 5, Chapter 2
• Allan Johnson, CCNA
• Modified by Sean He, CCNP

2
The Address Crisis Solutions Overview
3
Solving the Depletion Crisis

• In 1992, IETF had two main concerns
• Class A is gone and Class B is almost gone
• Internet routing tables are huge!!
• Therefore, over the next several years they came
  up with solutions
• Route Summarization using CIDR Notation
• Variable Length Subnet Masking
• Private Addressing and NAT
• IP Unnumbered on WAN links
• IP version 6

4
CIDR Notation

• Classless Interdomain Routing is a method of
  representing an IP address and its subnet mask
  with a network prefix and bitmask.
• For example 192.168.50.0/18
• What do you think the 18 tells you?
• 18 is the number of 1 bits in the subnet mask.
  Therefore, 255.255.192.0

5
Route Summarization

• Route Summarization is a method of representing
  multiple, contiguous subnets with one aggregated
  address.
• Without route summarization, the routing tables
  of the Internet wouldve collapsed back in the
  mid 1990s.
• Route summarization benefits include...
• More efficient routing
• reduced CPU usage
• reduced memory requirements

6
Route Summarization Example

• Your enterprise has four Class C addresses
• 199.100.0.0/24
• 199.100.1.0/24
• 199.100.2.0/24
• 199.100.3.0/24
• Notice these addresses are contiguous.
• With CIDR notation, we can represent all four
  addresses as 199.100.0.0/22. How?
• Because all four addresses have the first 22 bits
  in common (called a prefix).

7
Route Summarization Example

• Below is 199.100.0.0/22 worked out in binary.
• Being able to work at the bit level is crucial
  when supernetting to summarize a range of
  addresses.
• How does route summarization help reduce routing
  tables?

8
Route Summarization Example

• Your AS advertises a summarized route to your
  ISP.
• The ISP, in turn, advertises a further summarized
  route to the Internet, thereby reducing the
  Internets routing table.

9
Variable Length Subnet Masking
10
VLSM Overview

• You may have noticed in your CCNA studies that
  addressing a WAN link is often a waste of host
  addresses.
• VLSM allows you to subnet a subnet!
• WAN links only need 2 addresses for hosts.
• Therefore, using VLSM would yield a CIDR notation
  of /30 on WAN links.
• In addition, with the ip subnet-zero command
  enabled by default on Cisco IOS 12.0 and higher,
  you can now use subnet zero.

11
VLSM Example

• You have a small Class C network with 6 LANs 30
  hosts (192.168.1.x/27)
• NO MORE ADDRESSES for WAN links!!
• Solution Use subnet zero and subnet it further
• 192.168.1.4/30
• 192.168.1.8/30
• 192.168.1.12/30
• 192.168.1.16/30
• 192.168.1.20/30
• 192.168.1.24/30
• 192.168.1.28/30
• You now have enough addresses for 7 WAN links.

12
VLSM Example

• The graphic shows how you can have your 6 subnets
  with 30 hosts and still have subnets leftover for
  WAN links.
• The hub router would then summarize all the
  subnets as 192.168.1.0/24
• This simple demonstration of VLSM hides its true
  power... SCALABILITY!!
• Lets explore that power.

13
VLSM Routing Protocols

• Only the classless routing protocols shown in the
  table below support VLSM.

14
RIPv1 versus RIPv2

• RIPv1...
• does not send subnet mask information
• the receiving router applies its subnet mask or
  the default
• broadcasts its updates
• does not support authentication
• RIPv2...
• supports VLSM
• multicasts its updates
• supports authentication
• However, RIPv2 is still limited to 15 hops and
  only considers hops as its metric.
• Configuring RIPv2...
• Router(config)router rip
• Router(config-router)version 2

15
Addressing Solutions IPv6
16
Private Addressing NAT

• As discussed earlier, private IP addresses cannot
  exist on the Internet.
• Therefore, we use Name Address Translation (NAT)
  to dynamically give packets a real IP address.
• ISPs will only give you a limited number of real
  IP addresses (if any!). So NAT configuration also
  includes the ability to overload a real IP.
• The purpose of NAT overloading is to allow
  multiple local inside addresses to share a single
  global outside address.
• This is done by tracking source ports from the
  transport layer. As packets leave, not only do
  they get a real IP but are also tagged with a
  port number to identify the session (and host) as
  packets return from the destination.
• For more detail on NAT, review Semester 6s
  Chapter 11 devoted to the subject. We will not
  configure NAT this semester.

17
IP Unnumbered

• IP Unnumbered is used to conserve more space on
  WAN links.
• Serial interfaces borrow an IP address from
  another interface (typically a LAN interface)
• Rules for using IP unnumbered
• Only point-to-point serial interfaces
• Both sides must belong to the same major network
  with the same subnet mask, or different major
  networks with default subnet masks
• Drawbacks to using IP unnumbered
• Cannot ping the interface
• Cannot boot a network IOS image over interface
• Cannot use IP security
• Configuring IP Unnumbered
• Router(config)interface s0
• Router(config-if)ip unnumbered e0

18
DHCP Easy IP

• Hosts configured to dynamically obtain their IP
  addresses will send a DHCP broadcast upon
  booting.
• Configuring DHCP (Be sure to do Interactive Lab
  2.8.3)
• Router(config)ip dhcp excluded-address
  address_range
• Specifies a range of addresses to be excluded
  from the dhcp pool
• Router(config)ip dhcp pool pool_name
• Router(dhcp-config)network network_addresssubn
  et_mask
• Defines the name of the dhcp pool and the address
  to be used to assign IPs
• Router(dhcp-config)default-router
  router_address1router_address8
• Defines up to 8 routers from which the host can
  get IP addresses
• Ciscos Easy IP
• Plug and Play routing that allows a remote
  router to get a real IP address from the ISP
• Then the remote router uses DHCP/NAT to provide
  access to internal LAN clients.

19
Helper Addresses

• DHCP uses BootPs UDP port numbers 67 68 to
  broadcast for an IP addresses.
• Normally, routers will not forward UDP requests.
  This causes a problem if the local router is not
  the DHCP server.
• Therefore, we configure the hosts local router
  with a helper address to which it will forward
  UDP requests for services.
• On the interface where hosts requesting services
  are located
• Router(config-if)ip helper-address
  server_address
• Will forward the 8 UDP services below which
  includes DHCP
• For UDP services not included in the 8, use the
  global command
• Router(config)ip forward-protocol udp
  port_number
• For UDP services you want to exclude, use no in
  front of command

20
Internet Protocol, version 6

• IPv4 will eventually perish even with
• Subnetting (1985)
• VLSM (1987)
• CIDR (1993)
• The proliferation of IP addressable devices will
  eventually exceed IPv4s limit of 4 billion
  addresses.
• IPv6 is a 128 bit address. But because of the
  success of NAT and private IPs, IPv4 will not go
  away for some time.
• IPv6 will require network administrators to
  re-engineer their enterprises with new software
  and new hardware.