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Homework 6: Host Configuration

Host A attaches to the above institutional network for the first time. The Ethernet link comes
up. Suppose ARP, DNS and web caches are empty.
What minimum network configuration does Host A need to communicate with other hosts
within a subnet and off-subnet?
As hints to answer this question, ask yourself the following:
a) What address does a source host need to send and receive IP Packets? What range of
addresses are valid for this host given the topology above?
b) What address does a source host need to know to send packets to a destination host that is
not on the same subnet?
c) How does a source host know whether a destination is on or off-subnet?
d) How does a source host resolve a destination hostname to an IP address?
List the network configuration needed, and the valid values of this configuration based on the
information in the network topology above.
Answers:
a)
b)
c)
e)

Homework 7​: Communicating on and off-subnet

User on Host A uses the well-known ping program to ping the IP address of Host B. (In this case,
suppose the user knows the IP address of the destination they want to ping.) The ping utility
will request the ICMP layer to send an ICMP Echo Request to the destination IP address. Given
the network topology above what are the values of the fields in the various network headers of
the ICMP packet when it is sent. How does the host know this information? (Note some
information is hard-coded, some information is determined by the protocol, or has standard
“well-known” or default values, and other fields are dynamically learned via some protocol.
Indicate which fields fall into which category, and if dynamically learned, specify the protocol.)
ICMP Header:
Message Code:
Message Type:
IP Header:
Source IP:
Destination IP:
Time-to-live:
Encapsulated Protocol:
Ethernet Header:
Destination MAC:
Source MAC:
Encapsulated Protocol:

Homework 7 (continued):
Suppose the user on Host A now pings the DNS Server using its IP address. Which fields in the
above headers change and what are their values set to? How is this information determined?
ICMP Header (list changes if any):

IP Header (list changes if any):

Ethernet Header (list changes if any):

Homework 8: ​Email, DNS and NAT

User on Host A sends email to the local ISP mail server using a mail user agent with SMTP
enabled. First, the email program needs to know the hostname and IP address of the user’s mail
server.
a) Describe how the email program can use DNS resource records to determine the hostname
and IP address of the mail server to send the email to.

Homework 8 (cont):
When the mail server IP address is known, the mail user agent will open a TCP connection to
the mail server and use SMTP to send email. To access the mail server the packets will need to
be processed via the NAT on the router as the host has IP addresses from the private IP address
space.
b) Show what the TCP and IP header fields of the TCP and SMTP packets from Host A look like
before​ NAT:
TCP Source Port:
TCP Destination Port:
IP Source Address:
IP Destination Address:

c) Show what the TCP and IP header fields of the TCP and SMTP packets from Host A look like
after​ NAT:
TCP Source Port:
TCP Destination Port:
IP Source Address :
IP Destination Address:
.


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