Keithley 708B Reference Manual page 66

Models 707B and 708B Switching Matrix Reference Manual
The network ID must be unique among all network subnets that connect to the Internet or corporate
intranet. If the subnet will be connected to the public Internet, the network ID must be obtained from
the Network Information Center, which assigns and preserves unique IDs. In any case, each host ID
must be unique among all the hosts on the same network (which presumably has a unique network ID
number).
In the TCP/IP protocol, a subnet mask separates the network ID from the host ID. The subnet mask
looks like an IP address, but sets a data bit high for each position of the IP address that makes up the
network ID. Three different classes of network are defined with the IP address and subnet mask, as
shown in the following table.
Network classes defined by IP address and subnet mask combinations
Network class
A
B
C
In the IP address format, "n" is a network ID position, and "h" is a host ID position. For simplicity, the
first byte definition has been omitted from the table.
Class C networks are the most common and use the subnet mask 255.255.255.0. The first three
bytes are the network ID number and the last byte is the host ID on the network. Host ID numbers 1
through 254 are available for assignment. All hosts on the same isolated network must have the same
subnet mask. As a general rule, the top and bottom host numbers are reserved. The top one
(nnn.nnn.nnn.255) is the broadcast address and the bottom one (nnn.nnn.nnn.0) is shorthand for the
whole subnet.
DNS
The domain naming system (DNS) is a protocol that provides a way to associate a user-friendly name
to an IP address. For example, while few people know the IP address of Google's website, everyone
knows www.google.com. When you enter this URL into an Internet browser, the DNS on the network
looks up the URL and translates it to the IP address for the Google website. This is invisible to the
user.
For DNS to work, there must be a DNS server on the network and the correct IP address for that
server must be configured in the computer. Some LAN instruments support DNS — if so, the IP
address for the DNS server must be configured as well as the instrument IP address. Instruments,
especially LXI instruments, also have to show a valid hostname on their LXI LAN Welcome and IP
Configuration pages, and they need to use the DNS to validate that any hostname they display is
valid. If they fail to validate a hostname, they must display the IP address for the instrument or a blank
hostname in the hostname field on the web page.
DNS requires a network administrator that can update the database in the DNS server with any host
name and IP address combinations, so it is not usually suitable for instrumentation setups.
707B-901-01 Rev. A / August 2010
IP address Subnet mask
nnn.hhh.hhh.hhh 255.0.0.0
nnn.nnn.hhh.hhh 255.255.0.0
nnn.nnn.nnn.hhh 255.255.255.0
Available subnets
126
16384
2097151
NOTE
Section 2: General operation
Available hosts
16777214
65534
254
2-47