Thought IP Data Encapsulation hard to master? Approaching the IPX side
of the shop with not a little awe and trepidation?
Be calm. This truly is "elegant" (a Novell buzzword, in case
you don't already know).
Novell defines
encapsulation as "the technique used by protocols in which a
layer adds header information to the Protocol Data Unit (PDU) from the
preceding layer. Also used in enveloping one protocol inside another for
transmission, for example, Internet Protocol (IP) inside IPX."
Internetwork Packet exchange (IPX) was developed by Novell in the mid
1980s. It was based on the XNS protocol developed by Xerox. IPX was the
dominant LAN protocol in the late 80's and early 90's.
Why learn IPX Data Encapsulation
What you're about to read really is no longer relevant to Novell's most
recent releases of NetWare. Beginning in 5.0, NetWare network no longer
was locked into IPX. However, there are at least two very good reasons
to know what follows:
Cisco's present CCNA does not test on NetWare 5 or better.
Though Novell insists it soon will no longer support its older NetWare
versions starting in March 2002 (see official listing here), you as
a tech are likely to run into legacy systems throughout your career.
Novell's older versions of NetWare could well be among those systems.
This is good stuff to know.
The IPX address
IPX has an 80-bit address. The first 32 bits represent the network address
while the remaining 48 bits represent the node address. The network's
administrator determines the network address while the node address comes
from the interface's MAC address. For a serial interface, the node address
is borrowed from the first LAN interface. An IPX address is hexidecimal
with the format of network.node.node.node
Here's are some examples:
7c.0000.0ca1.4567 (network portion is 7c)
1ace44.0000.0c23.1231 (network portion is 1ace44)
4001231.0002.0D12.1241 (network portion is 4001231)
The Network
Of course, our study here is data encapsulation, or framing - how packets
get from upper layer protocols and building frames to across the network.
A quick recheck of the OSI Model (http://webopedia.internet.com/quick_ref/OSI_Layers.html)
will remind you frames exist at the Data Link, Layer 2, portion of the
OSI. With IPX, data encapsulation involves taking IPX datagrams from Layer
3 and building frames at Layer 2 in preparation for transmission over
the physical medium (Layer 1). You'll generally work with this in these
three physical network types:
Ethernet
Token Ring
FDDI
Novell NetWare IPX supports the following four encapsulation schemes:
Ethernet_802.3 aka, "802.3 raw or Novell Ethernet_802.3,"
this Novell proprietary is the initial encapsulation scheme use by Novell.
It includes the IEEE 802.3 (Ethernet) Length field but not an IEEE 802.2
(LLC) header. The IPX header immediately follows the 802.3 Length field.
This was the default until NetWare 3.11
Ethernet_802.2 aka, Novell_802.2, it's Logical Link Control (LLC).
This has been the default since NetWare 3.12
Ethernet_II aka Ethernet-II or ARPA, this includes the standard Ethernet
Version 2 header: Destination and Source Address fields followed by
an EtherType field. This supports TCP/IP and IPX
Ethernet_SNAP aka Ethernet_SNAP, this extends the IEEE 802.2 header
by providing type code like the Ethernet Version 2 specification. Supports
AppleTalk, TCP/IP and IPX
Conceptually, it all looks something like this:
The number of frame types will be determined by the network's environment.
If the router is configured to handle more than one frame type, then clients
configured to use different types will communicate just fine. However,
when configuring any IPX device on the network, including a router, frame
type assignments (as a general rule) should be consistent....
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