5. The ARP Table
• The ARP table is stored in area of Random-Access Memory on each
host.
• Such an area of memory is often called a cache. The ARP table is
often referred to as an ARP cache.
• Entries in the ARP table “age out.” They are removed from the table
after a period of inactivity.
6. Aging Out
• For Microsoft Windows hosts:
– Initial mappings have a 2-minute time-to-live.
– An entry that is used twice in 2 minutes is automatically given a 10-
minute time-to-live.
• For Unix/Linux hosts:
– Initial mappings have a 20 minute time-to-live.
7. Default Gateway
• The IP address of the router interface is stored
in the network configuration of the host.
• The source host compares the destination IP
address and its own IP address to determine if
the two IP addresses are located on the same
segment.
• If the receiving host is not on the same segment,
the source host sends the data using the actual
IP address of the destination and the MAC
address of the router.
• Either Proxy ARP or the Default Gateway must
be configured, or no traffic can leave the LAN.
8. Using a default gateway
• If the destination IP address is not on the same subnet (or network),
a computer must use the services of a router.
• Routers are sometimes called gateways for this reason.
• Sending computer checks for a default gateway in its TCP/IP
configuration.
• If no default gateway is installed, the sending computer cannot send
the message.
198.189.232.1
10. Web and HTTP
First some jargon
• Web page consists of objects
• Object can be HTML file, JPEG image, Java applet, audio file,…
• Most Web sites have a base HTML-file which includes several
referenced objects
• Each object is addressable by a URL
• Example URL:
http://www.cs.sfu.ca/CC/371/jcliu/index.htm
host name path name
11. HTTP overview
HTTP: hypertext transfer protocol
• Web’s application layer
protocol
• client/server model
– client: browser that
requests, receives,
“displays” Web objects
– server: Web server sends
objects in response to
requests
• HTTP 1.0: RFC 1945
• HTTP 1.1: RFC 2068
PC running
Explorer
Server
running
Apache Web
server
Mac running
Navigator
12. Cookies: keeping “state”
Many Web sites use
cookies
Example:
– You access Internet
always from same PC
– You visit a specific e-
commerce site for first
time
– When initial HTTP
requests arrives at site,
site creates a unique
ID and creates an
entry in backend
database for ID
13. Cookies: keeping “state”
Many major Web sites use
cookies
Four components:
1) cookie header line in the
HTTP response message
2) cookie header line in HTTP
request message
3) cookie file kept on user’s
host and managed by
user’s browser
4) back-end database at Web
site
