Learning by Doing: CISCO Certified Network ... - SCN Research

The Control (con) is 1 byte long and is usually set to a hexadecimal 03 for
Ethernet.
The 802.2 SNAP packet is composed of two fields:
The Organization Code (Org) is 3 bytes that are all usually set to zeros. In
hexadecimal that would be 000000.
The Type indicates what types of request will follow. This will be given in
hexadecimal. This field is usually 2 bytes. A 0800 in the type field indicates an
IP datagram will follow. A 0806 in the type field indicates an ARP request will
follow. A 0835 in the type field indicates a RARP request will follow. Current
type codes can be found at http://www.iana.org/numbers.html#
The Data is what it sounds like…it’s the “meat” of the information transmitted. For
“generic” Ethernet this can be as small as 46 bytes and up to 1500 bytes. The first part of
the data field contains the LLC information, then the SNAP information and finally the
IP header information. See the discussion below on the composition of the data field for
both types of Ethernet packets.
The Frame Check Sequence (FCS) is the CRC information for error control. This is 4
bytes in hexadecimal. There are many different error control calculations. (Is it a
coincidence there are many flavors of jello too?) I described one in an earlier lab using
unique prime numbers.
IP Data Field Composition
The “Internet Protocol” Standard was written by Postel in 1981 (ftp://ftp.isi.edu/innotes/rfc791.txt
). Geeze…it almost sounds like the egg came before the chicken? Well
anyway, the IP data field is begun with a header portion of 20 bytes unless options are
used.
Ver Hlen TOS Length
ID Flags FO TTL Prot HC SA DA Opt Data
Figure 3—IP Data Field Composition
The Version field is 4 bits. This is usually set for IP version 4 (IPv4) although IPv6 is
emerging quickly. IPv4 uses 4 bytes and IPv6 uses 6 bytes. In hexadecimal IPv4 is
denoted with a 45. IPv6 is denoted with 0x86dd.
The Header Length field is also 4 bits. It indicates how many 32-bit portions are in the
IP header (including options). The maximum is 60 bytes.
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