1. TCP/IP Internetworking (Part 2)
(February 8, 2016)
© Abdou Illia – Spring 2016

2. Border Router vs. Internal Router
Border routers connect different networks
In this case network x.x and network 60.x.x.x).
An “x” indicates anything
ISP Network
60.x.x.x
Subnet x
Subnet x
Subnet x
Internal
Router
Border
Corporate network x.x 2

3. Border Router vs. Internal Router (cont.)
Internal routers connect different subnets in a network
In this case, subnets: x, x, and x.
ISP Network
60.x.x.x
Subnet x
Subnet x
Subnet x
Internal
Router
Border
Corporate network x.x 3

4. IP Routing Routing Matches Host 60.3.47.129
Router A
Routing
Matches
Host
Because of multiple alternative routes in router meshes,
routers may have several rows that match an IP address.
Routers must find All matches and then select the BEST ONE.
This is slow and therefore expensive compared to switching. 4

5. IP Address Range Governed by the route
IP Routing (cont.)
The Routing Table
Each router has a routing table that it uses to make routing decisions
Routing Table Rows
Each row represents a route for a RANGE of IP addresses—often a network or subnet
All packets with destination addresses in this range are routed according to that row
Route
IP Address Range Governed by the route
Metric
Next-Hop
Router 1
60.3.x.x 9 B 5

6. IP Routing (cont.) The Routing Table Routing Table Columns
Row (route) number: Not in real routing tables
IP address range governed by the row
Metric for the quality of the route (cost, speed, etc.)
Next-hop router that should get the packet next if the row is selected as the best match
Route
IP Address
Range
Metric
Next-Hop
Router 1
60.3.x.x 9 B 2
x.x 2 B 6

7. IP Routing (cont.) A Routing Decision
The router looks at the destination IP address in an arriving packet (in this case, ).
1. The router determines which rows match (have an IP address range containing the packet’s destination IP address)
The router must check ALL rows for possible matches
Route
IP Address
Range
Metric
Next-Hop
Router
Arriving Packet 1
60.3.x.x 9 B
Match 2
x.x 2 B
No Match 3 x 8 C
Match 7

8. IP Routing (cont.) A Routing Decision
2. After finding all matches, the router then determines the BEST MATCH row
2A. Selects the row with the LONGEST MATCH
60.3.x.x has 16 bits of match
x has 24 bits of match so is a better match
2B. If two or more rows tie for the longest match, router uses the METRIC column value
If cost, lowest metric value is best
If speed, highest metric value is best
Etc. 8

9. IP Routing (cont.) A Routing Decision
3. After selecting the best-match row, the router sends the packet on to the next-hop router indicated in the best-match row—Next-Hop Router C in this example.
Route
IP Address
Range
Metric
Next-Hop
Router
Send Packet out to NHR B 1
60.3.x.x 9 B 2
x.x 2 B 3 x 8 C
Best-Match Row 9

10. Vertical Communication on Routers
Internet Layer Process
Packet
Port 1 DL
Port 2 DL
Port 3 DL
Port 4 DL
Decapsulation
Frame
PHY
PHY
PHY
PHY
Notes:
Router R1 receives frame in Port 1.
Port 1 Data Link decapsulates the IP packet.
Port 1 Data Link passes packet to internet Layer.

11. Vertical Communication on RoutersB
Internet Layer Process
Packet
Port 1 DL
Port 2 DL
Port 3 DL
Port 4 DL
Encapsulation
Frame
PHY
PHY
PHY
PHY
Router 2
Internet layer sends packet out on Port 4.
Data Link process on Port 4 encapsulates packet in a DL frame.
Data Link process passes frame to Port 4 PHY.

12. Summary Questions (Part 1)
How many layers are there in a router?
Can a router be a software program?
Suppose that Computer 1 sends a message to Computer 2. Assume that there are two routers (R1 and R2) along the route that leads to Computer 2. Assume that a frame from the message is received by R1 in Port 2. Which of the following will happen next?
The Data Link layer process in Port 1 will de-encapsulate the IP packet from the frame
The Physical layer will pass the frame to the Data Link layer process in Port 2
The Data Link layer process in Port 2 will de-encapsulate the IP packet from the frame
None of the above

13. IP Addresses Network Webserver Software Browser Packet Packet Route
Router
Webserver
(Host)
IP Address=
Host name=eiu.edu
User PC
(Host)
IP Address=

14. IP Address IP is a connectionless protocol
IP address is like postal addresses
Postal addresses are hierarchical: state, city, postal zone, street, house address
IP Addresses have the following hierarchy
Network number (tells what network the host is on)
Subnet number (tells what segment of network the host is on)
Computer number (identifies a particular computer on the segment)
Routers look at network part (and segment part for some) to make routing decisions
Final router looks at Host part

15. Hierarchical IP Address
Network Part (not always 16 bits)
Subnet Part (not always 8 bits)
Host Part (not always 8 bits)
Total always is 32 bits.
The Internet 13
EIU Network (139.67)
School of Business Subnet
(130)
Host 13

16. IP Address notations IP addresses 127.18.47.145 127.47.17.47
Are really strings of 32 bits (1s and 0s)
Example:
Usually represented by four number segments separated by dots: dotted decimal notation
Example:

17. IP Address structure Each IP Address has two main parts:
Each Organization is given the Network Part by an IP address Registrar (see
For Eastern Illinois University, this is
All IP Addresses for Eastern’s computers begin with that Network Part
Network Part
Local Part
IP Address
139.67
American Registry for Internet Numbers
Check EIU’s IP registration info

18. IP Address structure Network Parts is 8 to 24 bits long Local Part
For Eastern, it is 16 bits long
16 bits is only an example
Local Part
Total address is 32 bits, so if the network part is 8 bits, the local part is 24 bits
Network Part
(8 to 24 bits)

19. IP Address (32 bits total)
Assigning Parts
Most Organizations have multiple segments within the organizational network
So, usually Local Part is broken in two parts
a Segment Part to represent each segment
Remaining Bits are the Host Part, designating a particular computer on that segment
Local Part
Network Part
Segment Part
Host Part
IP Address (32 bits total)

20. Network classes Classful IP Addressing
Question: Determine the number of maximum networks and hosts for each class.
Classful IP Addressing
Classless IP Addressing (since 1993)
Most networks too big for "class C“. Therefore received a "class B" set of addresses instead.
With the rapid growth of the Internet, there is a shortage of class B addresses
Classful IP addressing is replaced by Classless IP addressing
e.g., instead of a full class B, business gets "a set of /16 addresses"
Class
Leftmost
bits
Network Part
Length
Address range
# of networks
# of host/ network
Class A
0xxx
8 bits
0.x.x.x to 127.x.x.x
Class B
10xx
16 bits
128.0.x.x to x.x
Class C
110x
24 bits
x to x

21. Spoofed Source IP Address
IP Address Spoofing
IP address spoofing is sending a message with a false IP address with the intent to mislead the receiving device and gain access
1. Trust Relationship
Trusted Server
Victim Server
From:
To: 2.
Spoofed Source IP Address
is used.
Attacker’s Client PC
Reasons for IP spoofing:
Anonymity
Exploiting trust relationship