Determine Internet connection with PING or Telnet trouble shooting

Internet TCP/IP connection is two-way communication.
You will be sending information to the SMTP mail server
port 25 and it will be sending status information back...

I can't seem to be able to broadcast any emails?

If your PCs modem is a direct Internet connection and you can't broadcast any emails... See: PING for Network trouble shooting or Telnet to determine if the problem is with your Internet connection.

Mail Server
If you are using your own mail server program (sending mails from your PCs). See: Mail Server FAQs?

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How do I broadcast email through a firewall?

Firewall - If your PCs modem or internet connection is connected through a (hardware) router or firewall.

Sending e-mail through a firewall requires the firewall to be setup to allow email messages a free path to travel.

Internet TCP/IP connection is two-way communication. You will be sending information to the mail server and it will be sending status information back and two ports will be involved in the conversation: the client's port and the server's port.

All ports below 1025 are reserved for use by specific protocols. Port 25 is reserved for SMTP mail servers, port 110 for POP3 mail servers and port 143 for IMAP4 mail servers.

Since the client is not a mail server - it can't use port 25 therefore, the client system will auto select a port above 1024 and the mail server will communicate on ports 25, 110 and 143.

Your firewall will have to allow outbound traffic from any internal port greater than 1024 to external ports 25, 110 and 143. Your firewall will also need to allow inbound traffic from external ports 25, 110 and 143 to any internal port greater than 1024.

Your default server port can be changed by specifying the desired port number in the MailPort property for SMTP or POP3 and IMAP4, plus adjusting the mail server to listen on those ports (client port can't be changed).

NAT - Network Address Translation, allows a router (which represent an entire local area network to the Internet) to act as a single IP address. And all traffic leaving a LAN (Local Area Network) will be as if it originated from a global IP address and all traffic coming into the LAN will also be using the same global IP address.

Therefore, since a LAN cannot be seen from the outside (all workstation hidden from the Internet), registration of the internal IP addresses or Local IP address-(below) will not be necessary and the normal range of private addresses allocated are: 10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255
See Private Network

NAT - When a user inside the LAN sends a packet to the outside (Internet), NAT interface changes the internal address to a global IP address and cache them--and when a response comes back from the Internet, NAT looks in the cache and switches the addresses back--if not in cache, the packet is dropped.

Therefore, any LAN workstation connected to the Internet needs an IP address, LAN router IP address, IP address name server and NAT subnet mask interface. And the LAN system must also set-up a service table of fixed IP address mappings that allows packets to originate from the outside.

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Local IP Address
When 2 computers try to communicate, an ARP request is initiated--if the IP address is on the local network, it (the source host) checks its ARP cache to see if it has the hardware MAC address of the receiving host--if not, a broadcast is sent to all local hosts.

If the receiving host finds that the IP address of the source host matches its own--it sends a reply to the source host with its hardware address. When received by the source host, its ARP cache is updated to include it--if no hosts respond to the broadcast then the request is discarded.

ARP - Address Resolution Protocol, translates an IP packet address into a MAC layer address-(below) so that frames can be delivered to a LAN destination workstation (LAN does not use IP addresses but MAC layer address). ARP are dynamic or can be static and if a dynamic entry is not used within 2 minutes, it gets deleted--if used it remains for 10 minutes, whereas, a static entry will remains until the machine is rebooted or a dynamic entry is received to overwrite it.

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Remote IP Address - When the destination address is checked-out to be a remote host--the source host checks the local routing table for a path to the receiving host--if a path is not found then a broadcast is sent to the gateway (router)--it then respond with its hardware address and then the packet is sent to the router.

Basically, the router follows the same pattern--by checking its cache for a path to the receiving host--if one is found, the packet is forwarded--if not, it sends a broadcast and waits for a reply from the host. It may again determine that it is a remote host and then the process repeats with a broadcast to the next router and so on--once the receiving host gets the request, it sends an ICMP echo request.

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A router - or gateway is a hardware device that forwards packets from one network to another. IP uses a routing table to determine which networks that packets can be forwarded to and will only forward them to networks that it has been configured to do so.

When a packet is to be sent, IP determines whether or not the IP address is local or remote. If it is local, it forward the packet. If it is remote, it consults the routing table to determine the path to the remote host. If there is no entry in the routing table, then the default gateway is used instead.

The routing table is checked at the router and the process repeats. A packet can be forwarded from router to router. Each one of these steps is called a hop --if a route to the remote host is never found then an error message will return. Routing can be either static or dynamic. Static routing involves manually programming the routing table into the router.

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MAC (Media Access Control) manages one of the sublayers handles access to shared media.

MAC address (are 6 bytes long and are controlled by the IEEE). It is a standardized data link layer address that is required for every port or device that connects to a LAN. Other devices in the network use these addresses to locate specific ports in the network and to create and update routing tables and data structures.

MAC addresses are also known as;

Hardware address
MAC-layer address
Physical address.
ethernet addre

A MAC layer address looks is something like this: 08:00:05:81:79:66. and it should not be confused with an IP address that looks something like this: 121.006.62.06 they are not the same and perform different functions.

MAC address can be displayed for Windows NT users by opening a command shell (DOS prompt) and entering ipconfig /all -- to display a text list of PCs network info.

Win95 users can get the MAC address by opening a command shell (DOS prompt) and entering net diag /s and you will be asked for the computer Netbios name and enter key--the MAC address displayed next Permanent Node Name

Win95 users can get the MAC address by opening a command shell (DOS prompt) and entering winipcfg if TCP/IP protocols are already installed.

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MTU Maximum Transmission Unit - is the default packet Windows uses to negotiate when a connection is opened between two computers, they must agree on an MTU by comparing MTU and selecting the smaller of the two. If it is set too large for routers, it is then fragmented into packet size and can take double the amount of time it takes to send a single packet.

UART Universal Asynchronous Receiver Transmitter Data flow between the serial port and the modem is governed by a UART chip, which can contain no data buffers, single-byte data buffers or 16-byte data buffers. All data flows between the serial port and the modem via the UART

RWIN (TCP Receive Window) determines how much data the receiving computer is prepared to receive. RWIN value that is set too high will result in greater data loss if the packet is lost or damaged in transit. RWIN value that is set too low will produce poor throughput.

MSS (Maximum Segment Size) is the largest segment of TCP data that Winsock is prepared to receive on a particular connection. When the TCP connection is initially established, both use the minimum of each other's MSS value. If the MSS is too low, the data/header ratio will be low. If the MSS is too high, will lead to large IP datagram and the packets will tend to fragment in transit where other networks may be having smaller MTU. Hence, performance can be increase by reducing MSS value.

Since packet headers are normally 40 bytes in length, your MSS value should always be set to MTU value minus 40 (i.e. if your MTU is 1500 the set MSS to 1460) at least 40 less than the value of the MTU

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eMail Broadcast FAQ

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Firewall a hardware or software device which regulates data flowing into and out-of a private network.

TCP/IP Transmission Control Protocol Internet Protocol
The primary protocol used on the Internet.

Protocol a method of sending and receiving data.

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ICMP - Internet Control Message Protocol is used by routers and nodes to perform error reporting for Internet Protocol. It report errors such as invalid IP address, invalid port address, and that the packet has hopped too many times.

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TTL-Time To Live, is a field in the IP header which indicates how long a packet should be allowed to survive before it is discarded.

TTL essentially determines the maximum number of hops permitted.