Router
Host 1
Routing table for router 1
Network
Next router
Distance
A
Direct connection
0
B
Direct connection
0
C
Direct connection
0
D
Router 3
1
Network B
Router 2
Routing table for router 2
Network
Next router
Distance
A
Router 1
1
B
Direct connection
0
C
Direct connection
0
D
Router 3
1
Network C
Router 1
Network A
IP packet
Router 3
Source IP address
Destination IP address
Network A/Host 1
Network D/Host 2
Other control/information data
There are two paths when transmitting packets from Host 1
to Host 2. The router selects the optimum path (shortest
path), and the packets are carried in the arrow direction.
Routing table for router 3
Network
Next router
Distance
A
Router 1
1
B
Router 2
1
C
Direct connection
0
D
Direct connection
0
Host 2
Network D
Protocols used for e-mail
3
The protocols used for e-mail are summarized below.
Protocol
Reference
SMTP
A protocol for sending e-mail. Used when sending e-mail between mail
servers, or from a mail client to a mail server.
POP3
A protocol for retrieving e-mail. Retrieves all newly arrived e-mail that
has been stored on the mail server and is addressed to the user.
IMAP4
A protocol for retrieving e-mail. Can selectively retrieve e-mail that is
stored on the mail server.
IMAP4
Abbreviation for “Internet Message Access Protocol version 4.”
Description
Example
Protocol for sending e-mail from A to D
D
Internet
Mail addressed
to D
A
①
Router
Send by SMTP
② Transfer
by SMTP
Router
③
Retrieve by
POP3 or IMAP4
Mail addressed to D
B
Mailbox for D
Mail addressed
to D
Mail server
Mail server for A
C
Mail server
Mail server for D
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