host
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... topology, rather the interpretation of
addresses is hierarchical. In this two-level model, each host sees
its network as a single entity ...
... single entity; that is, the network may be treated
as a "black box" to which a set of hosts is connected.
...
... Limits of technologies: Most LAN technologies impose limits,
based on electrical parameters, on the number of hosts
connected, and on the total length of the cable. It is easy to
exceed these limits, especially those on cable length.
...
... Network congestion: It is possible for a small subset of the
hosts on a LAN to monopolize most of the bandwidth. A common
...
... LAN to monopolize most of the bandwidth. A common
solution to this problem is to divide the hosts into cliques of
high mutual communication, and put these cliques on separate
cables.
...
... Use a single network number for the entire organization, but
assign host numbers without regard to which LAN a host is on
...
... Use a single network number, and partition the host address
space by assigning subnet numbers to the LANs ...
... However, these changes are relatively minor, and once made, yield a
simple and efficient solution to the problem. Also, the approach
avoids any changes that would be incompatible with existing hosts on
non-subnetted networks.
...
...
Further, when appropriate design choices are made, it is possible for
hosts which believe they are on a non-subnetted network to be used on
a subnetted one, as explained in RFC-917 ...
... 917 [1]. This is useful when it
is not possible to modify some of the hosts to support subnets
explicitly, or when a gradual transition is preferred.
...
... Internet addresses to support subnets. Next it discusses changes to
host software to support subnets. Finally, it presents a procedures
for discovering what address ...
... network into a set of subnets,
and wants to assign host addresses: how should this be done?
Since there are minimal restrictions on the assignment of the
...
... subnet. However, this can also be viewed as a disadvantage:
it may cause problems for networks which have existing host
numbers that use arbitrary bits in the local address ...
... bits of local address to subnet and host numbers. Therefore, we
choose the address-mask scheme: it is the most flexible scheme,
...
... network. No further
structure is required for the <subnet-number> or <host-number>
fields. If the width of the <subnet-number> field is zero, then
...
... |1 0| NETWORK | SUBNET | Host Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
... with functional significance rather than as identifiers of
specific hosts. When such usage is called for, the address
zero is to be interpreted as meaning "this", as in "this
...
... network". The address of all ones are to be interpreted as
meaning "all", as in "all hosts". For example, the address
128.9.255.255 could be interpreted as meaning all hosts ...
... hosts". For example, the address
128.9.255.255 could be interpreted as meaning all hosts on
the network 128.9. Or, the address ...
... network 128.9. Or, the address 0.0.0.37 could be
interpreted as meaning host 37 on this network."
...
... Please note that there is no effect or new restriction on the
addresses of hosts on non-subnetted networks.
...
...
To support multiply-connected hosts, the code can be changed to
keep the "my_ip_addr" and "my_ip_mask" quantities on a
per-interface ...
... to which it is connected? The problem is analogous to several
other "bootstrapping" problems for Internet hosts: how a host
determines its own address ...
... other "bootstrapping" problems for Internet hosts: how a host
determines its own address, and how it locates a gateway ...
...
Hardwired information is that available to a host in isolation
from a network. It may be compiled-in, or (preferably) stored in
...
... broadcasting, a better
method is for the newly-booted host to broadcast a request for the
necessary information. For example, for the purpose of
...
... necessary information. For example, for the purpose of
determining its Internet address, a host may use the "Reverse
Address Resolution Protocol" (RARP) [4 ...
...
However, since a newly-booted host usually needs to gather several
facts (e.g., its IP address, the hardware address ...
... network.
The mechanisms designed to boot diskless workstations can also
load per-host specific configuration files that contain the
required information (e.g., see RFC-951draft ...
... 951draft [8]). It is possible, and
desirable, to obtain all the facts necessary to operate a host
from a boot server using only one broadcast message.
...
...
In the case where it is necessary for a host to find the address
mask as a separate operation the following mechanism is provided:
...
...
The intended use of these new ICMP messages is that a host,
when booting, broadcast an "Address ...
... Address Mask Request" message. A
gateway (or a host acting in lieu of a gateway) that receives
this message responds with an "Address ...
... this message responds with an "Address Mask Reply". If there
is no indication in the request which host sent it (i.e., the
IP Source Address is zero), the reply is broadcast ...
... IP Source Address is zero), the reply is broadcast as well.
The requesting host will hear the response, and from it
determine the address mask.
...
... Address Mask Reply" on any given LAN, there is no need for the
requesting host to match the responses it hears against the
request it sent; similarly, there is no problem if more than
one gateway ...
... request it sent; similarly, there is no problem if more than
one gateway responds. We assume that hosts reboot
infrequently, so the broadcast load on a network ...
...
One potential problem is what a host should do if it can not find
out the address mask, even after a reasonable number of tries.
...
... Internet
network number mask. Although this might later turn out to be
wrong, it will not prevent transmissions that would otherwise
succeed. It is possible for a host to recover from a wrong
choice: when a gateway comes up, it should broadcast ...
... broadcast an "Address
Mask Reply"; when a host receives such a message that disagrees
with its guess, it should change its mask to conform to the
received value. No host ...
... host receives such a message that disagrees
with its guess, it should change its mask to conform to the
received value. No host or gateway should send an "Address Mask
...
... to discover the address mask; it is perfectly reasonable for a
host with non-volatile storage to use stored information
(including a configuration file ...
... request was received.
If the requesting host does not know its own IP address, it may
leave the source field zero; the reply should then be
...
...
The most efficient method, and the one we recommend, is for a host
to first discover its own address (perhaps using "RARP ...
...
Suppose that 36.40.0.123 is a diskless workstation, and does not
know even its own host number. It could send the following
datagram:
...
... reply. Even so, the over use of broadcasts presents an
unnecessary load to all hosts on the subnet, and so the use of the
"anonymous" (0.0.0.0) source address ...
...
If broadcasting is not allowed, we assume that hosts have wired-in
information about neighbor gateways ...
... Mask: 255.255.252.0
In the diskless workstation case the host sends:
Source address ...
... Mask: 255.255.255.88.
In the diskless workstation case the host sends:
Source address ...
... datagrams when necessary, but whose existence is not known to
other hosts. Also called a "software repeater".
...
... Host Field ...