INTERNET PROTOCOL (IP) (see RFC 791) The Internet Protocol (IP) provides a frame for encapsulating other protocols like TCP and UDP. The IP header informs the recipient among other things of: the destination and source addresses of the packet, number of octets in the packet, whether the packet can be fragmented or not, how many hops can the packet traverse, the protocol that the packet carries, etc. The IP version currently utilizied is 4.
IP HEADER FORMAT OCTET 1 Version (4 bit)+IHL (4 bit) (VER, IHL) OCTET 2 Type of service (TOS) OCTET 3,4 Total Length (TOL) OCTET 5,6 Identification (ID) OCTET 7,8 Flags (3 bit)+Fragment Offset (13 bit) (FLG, FRO) OCTET 9 Time to Live (TTL) OCTET 10 Protocol (PRO) OCTET 11,12 Header Checksum (IP_SUM) OCTET 13,14,15,16 Source Address (SRC) OCTET 17,18,19,20 Destination Address (DEST) OCTET 21,22,23 Options (OPT) OCTET 24 Padding OCTET 25, 26 … Data In the example shown the IP packet is encapsulated in a PPP frame including the flag sequence (7E), address (FF 03), protocol (00 21) and FCS (0B 81). In turn, the IP packet encapsulates a UDP message. Example: 7E FF 03 00 21 45 00 00 40 00 01 00 00 3C 11 E0 31 CE D9 8F 1F C7 B6 78 CB 04 63 00 35 00 2C AB DA 00 01 01 00 00 01 00 00 00 00 00 00 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 0B 81 7E Start 7E Address FF 03 SEP 00 21 IP Header 45 00 00 40 00 01 00 00 3C 11 E0 31 CE D9 8F 1F C7 B6 78 CB Data 04 89 00 35 00 2C AB B4 00 01 01 00 00 01 00 00 00 00 00 00 04 70 6F 70 64 02 69 78 06 6E 65 74 63 6F 6D 03 63 6F 6D 00 00 01 00 01 FCS 0B 81 Stop 7E IP Header VER=4 IHL=5 TOS=0 TOL=64 ID=1 FLG=00 FRO=00 TTL=60 PRO=17 IP_SUM=E031 SRC=206.217.143.31. DEST=199.182.120.203. OPT=00000000
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