Specifications

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8.2.1 Using multiple ISPs
The fallback feature is great but it adds some complexity in setting up the network. Each provider issues a
different IP address and uses different DNS and gateway servers. The router hides these differences from
the local machines. As far as they are concerned the router is the gateway and DNS sever.
Another problem concerns sending email. This is not an issue if you use web-based email. If you use a
POP/SMTP mail client connecting through different ISPs may prevent you from sending mail. As the
Internet has become more popular some of the assumption made in the initial design have come up short.
Mass mailers have exploited the lack of SMTP security to inundate users with unsolicited email called
SPAM. The SMTP mail server cheerfully accepts all outgoing mail sent to it and delivers it to everyone on
the address list. Spammers love this, all they need is an open SMTP mail server and they are in business.
As a counter measure most ISP’s reject mail unless it is sent from within their network. This restricts
outgoing mail to users that are currently logged in giving the ISP some control over Spam. This is not a
problem if one has a single email account provided by the ISP. However if you use multiple email accounts
sending mail independent of the connection can be a problem. See section 11.4 SMTP SPAM mitigation
for more details.
Our hosting service uses SMTP authentication. Neither Verizon DSL nor our dialup ISP block SMTP port
25 this allows us to send mail securely regardless of how we connect.
Usenet may also be a problem of the ISP auto authenticates. In that case you will not be able to access news
when you use an alternative ISP.
8.3 LAN Address Assignment
Each device on the network requires an IP address. The LAN uses reserved private addresses. These
addresses are not used on the Internet therefore they do not need to be coordinated with other Internet users.
However they still must be coordinated within your own network since local addresses cannot overlap. The
Multitech router has the flexibility to use dynamic, pseudo static, or static addresses.
8.3.1 Dynamic
In most cases dynamic address assignment is convenient. When a new machine is plugged in the DHCP
server, built into the router, assigns it an address. Once the device has an address it is able to use the LAN.
The DHCP server assigns several other critical numbers, subnet mask gateway address and DNS address.
As discussed previously the subnet mask defines how the address should be interpreted. Only machines on
the same subnet can directly communicate. The gateway address is where packets that cannot be delivered
locally are sent. In our case the gateway is the Multitech router. The router decides how to deliver the
packet. Since only a single connection exists between our network and the ISP routing is trivial. The router
simply forwards all packets to the gateway address assigned by the ISP. Local host name resolution is done
within Windows. If Windows cannot resolve a host name it forwards the request to the router. The router in
turn forwards the request to the DNS address specified by the ISP.
8.3.2 Pseudo Static
For some devices, such as servers, dynamic addresses are a problem. For example the binding to the print
server is by its IP address, it does not have a name. That means if the address changes each client has to be
reconfigured. A solution is to create a pseudo static address. The address is issued by the DHCP server but
bound to the client’s Ethernet MAC address. This is more convenient then setting IP addresses manually
and making sure they do not conflict with previously assigned addresses or the DHCP pool.
This is especially useful for network appliances that do not include user interface.