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[Chapter 2] 2.4 Internet Routing Architecture

[Chapter 2] 2.4 Internet Routing Architecture

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[Chapter 2] 2.4 Internet Routing Architecture

The new routing model is based on co-equal collections of autonomous systems, called routing
domains. Routing domains exchange routing information with other domains using Border Gateway
Protocol (BGP). Each routing domain processes the information it receives from other domains.
Unlike the hierarchical model, this model does not depend on a single core system to choose the
"best" routes. Each routing domain does this processing for itself; therefore, this model is more
expandable. Figure 2.4 represents this model with three intersecting circles. Each circle is a routing
domain. The overlapping areas are border areas, where routing information is shared. The domains
share information, but do not rely on any one system to provide all routing information.
Figure 2.4: Routing domains

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[Chapter 2] 2.4 Internet Routing Architecture

The problem with this model is: how are "best" routes determined in a global network if there is no
central routing authority, like the core, that is trusted to determine the "best" routes? In the days of the
NSFNET, the policy routing database (PRDB) was used to determine whether the reachability
information advertised by an autonomous system was valid. But now, even the NSFNET does not
play a central role.
To fill this void, NSF created the Routing Arbiter (RA) servers when it created the Network Access
Points (NAPs) that replaced the role of the NSFNET. A route arbiter is located at each NAP. The
server provides access to the Routing Arbiter Database (RADB), which replaced the PRDB. Internet
Service Providers can query servers to validate the reachability information advertised by an
autonomous system.
Many ISPs do not use the route servers. Instead they depend on formal and informal bilateral
agreements. In essence, two ISPs get together and decide what reachability information each will
accept from the other. They create, in effect, local routing policies. This is a slow manual process that
probably will not be flexible enough for a rapidly growing Internet.
The RADB is only part of the Internet Routing Registry (IRR). As befits a distributed routing
architecture, there are multiple organizations that validate and register routing information. Europeans
were the pioneers in this. The Reseaux IP Europeens (RIPE) Network Control Center (NCC) provides
the routing registry for European IP networks. Big network carriers, like MCI and ANS, provide
registries for their customers. All of the registries share a common format based on the RIPE-181
Creating an effective routing architecture continues to be a major challenge for the Internet that will
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[Chapter 2] 2.4 Internet Routing Architecture

certainly evolve over time. No matter how it is derived, eventually the routing information winds up
in your local gateway, where it is used by IP to make routing decisions.

Previous: 2.3 Subnets
2.3 Subnets

TCP/IP Network
Book Index

Next: 2.5 The Routing
2.5 The Routing Table

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[Chapter 2] 2.3 Subnets

Previous: 2.2 The IP

Chapter 2
Delivering the Data

Next: 2.4 Internet Routing

2.3 Subnets
The structure of an IP address can be locally modified by using host address bits as additional network
address bits. Essentially, the "dividing line" between network address bits and host address bits is
moved, creating additional networks, but reducing the maximum number of hosts that can belong to
each network. These newly designated network bits define a network within the larger network, called
a subnet.
Organizations usually decide to subnet in order to overcome topological or organizational problems.
Subnetting allows decentralized management of host addressing. With the standard addressing
scheme, a central administrator is responsible for managing host addresses for the entire network. By
subnetting, the administrator can delegate address assignment to smaller organizations within the
overall organization - which may be a political expedient, if not a technical requirement. If you don't
want to deal with the data processing department, assign them their own subnet and let them manage
it themselves.
Subnetting can also be used to overcome hardware differences and distance limitations. IP routers can
link dissimilar physical networks together, but only if each physical network has its own unique
network address. Subnetting divides a single network address into many unique subnet addresses, so
that each physical network can have its own unique address.
A subnet is defined by changing the bit mask of the IP address. A subnet mask functions in the same
way as a normal address mask: an "on" bit is interpreted as a network bit; an "off" bit belongs to the
host part of the address. The difference is that a subnet mask is only used locally. In the outside world
the address is still interpreted as a standard IP address.
Assume we have been assigned network address The subnet mask associated with that
address is The most commonly used subnet mask, and the one we use in most of our
examples, extends the network portion of the address by an additional byte, e.g., The
subnet mask that does this is; all bits on in the first three bytes, and all bits off in the
last byte. The first two bytes define the original network; the third byte defines the the subnet address;
the fourth byte defines the host on that subnet.
Many network administrators prefer byte-oriented masks because they are easy to read and understand
when addresses are written in dotted decimal notation. However, limiting subnet masks to byte
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[Chapter 2] 2.3 Subnets

boundaries does not take advantage of their true power. The subnet mask is bit-oriented. We could
subdivide into 16 subnets with the mask, i.e. Applying
this mask defines the four high-order bits of the third byte as the subnet part of the address, and the
remaining 12 bits - four bits of the third byte and all of the fourth byte - as the host portion of the
address. This creates 16 subnets that each contain more than four thousand host addresses, which may
well be better suited to our network and organization. For example, we may have a small number of
large subdivisions. Table 2.1 shows the subnets and host addresses produced by applying this subnet
masks to network address
Table 2.1: Effect of a Subnet Mask
Network Number First Address Last Address
You don't have to manually calculate a table like Table 2.1 to know what subnets and host addresses
are produced by a subnet mask. The calculations have already been done for you. RFC 1878 lists all
possible subnet masks and the valid addresses they produce.
Organizations have been discouraged from subnetting class C addresses because of the fear that
subnetting reduces the number of host addresses to increase the number of network addresses. A class
C network is limited to fewer than 255 host addresses. Further limiting the number of hosts would
reduce the utility of a class C address. The mask divides a class C address into four
subnets of 64 host addresses. The fear is that the subnet address of all 0s and the subnet address of all
1s will not be usable. This leaves only two subnets; and because host addresses of all 1s and all 0s are
also unusable, the remaining two subnets can only address 62 hosts. Therefore the address space of
this class C network number is reduced from 254 hosts to 124 hosts. The fear of subnetting class C
addresses is no longer justified.
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