Kaspersky lab internet security for mac. Question: Determine IP and MAC header information for a data packet. IP ADDRESS MAC ADDRESS CLIENT A. IP ADDRESS MAC ADDRESS CLIENT A. Determine IP and MAC header information for a data packet. IP Header Format Unlike the post office, a router or computer cannot determine the size of a package without additional information. A person can look at a letter or box and determine how big it is, but a router cannot. Therefore, additional information is required at the IP layer, in addition to the source and destination IP addresses. Figure 3-12 is a logical representation of the information that is used at the IP layer to enable the delivery of electronic data. This information is called a header, and is analogous to the addressing information on an envelope. A header contains the information required to route data on the Internet, and has the same format regardless of the type of data being sent. ![]() This is the same for an envelope where the address format is the same regardless of the type of letter being sent. IP Header Format The fields in the IP header and their descriptions are • VersionA 4-bit field that identifies the IP version being used. The current version is 4, and this version is referred to as IPv4. • LengthA 4-bit field containing the length of the IP header in 32-bit increments. The minimum length of an IP header is 20 bytes, or five 32-bit increments. The maximum length of an IP header is 24 bytes, or six 32-bit increments. Therefore, the header length field should contain either 5 or 6. • Type of Service (ToS)The 8-bit ToS uses 3 bits for IP Precedence, 4 bits for ToS with the last bit not being used. The 4-bit ToS field, although defined, has never been used. • IP Precedence A 3-bit field used to identify the level of service a packet receives in the network. • Differentiated Services Code Point (DSCP)A 6-bit field used to identify the level of service a packet receives in the network. DSCP is a 3-bit expansion of IP precedence with the elimination of the ToS bits. • Total LengthSpecifies the length of the IP packet that includes the IP header and the user data. The length field is 2 bytes, so the maximum size of an IP packet is 2 16 1 or 65,535 bytes. • Identifier, Flags, and Fragment OffsetAs an IP packet moves through the Internet, it might need to cross a route that cannot handle the size of the packet. The packet will be divided, or fragmented, into smaller packets and reassembled later. These fields are used to fragment and reassemble packets. • Time to Live (TTL)It is possible for an IP packet to roam aimlessly around the Internet. If there is a routing problem or a routing loop, then you don't want packets to be forwarded forever. A routing loop is when a packet is continually routed through the same routers over and over. The TTL field is initially set to a number and decremented by every router that is passed through. When TTL reaches 0 the packet is discarded. • ProtocolIn the layered protocol model, the layer that determines which application the data is from or which application the data is for is indicated using the Protocol field. This field does not identify the application, but identifies a protocol that sits above the IP layer that is used for application identification. • Header ChecksumA value calculated based on the contents of the IP header. Used to determine if any errors have been introduced during transmission. • Source IP Address32-bit IP address of the sender. • Destination IP Address32-bit IP address of the intended recipient. • Options and PaddingA field that varies in length from 0 to a multiple of 32-bits. ![]() If the option values are not a multiple of 32-bits, 0s are added or padded to ensure this field contains a multiple of 32 bits. The IP Precedence field can be used to prioritize IP traffic. (See Table 3-9.) This is the same as the postal system having different classes of mail such as priority, overnight, and 2-day delivery. Routers can choose to use this field to give preferential treatment to certain types of IP traffic. Table 3-9 IP Precedence Values Precedence Value Meaning 000 (0) Routine or Best Effort 001 (1) Priority 010 (2) Immediate 011 (3) Flash Precedence Value Meaning 100 (4) Flash Override 101 (5) Critical 110 (6) Internetwork Control 111 (7) Network Control The ToS bits were originally designed to influence the delivery of data based on delay, throughput, reliability and cost. (See Table 3-10.) They are usually not used and are therefore set to zero. Table 3-10 Type of Service Values ToS Value Meaning 0000 (0) Normal Delivery 0001 (1) Minimize Cost 0010 (2) Maximize Reliability 0100 (4) Maximize Throughput 1000 (8) Minimize Delay The IP Precedence field can have 8 or 2 3 possible values. Routers use two of these values, 6 and 7, for routing protocol traffic. That leaves six values that can be used to prioritize user traffic. Because the ToS bits are typically not used, the IP Precedence field can be extended from 3 to 6 bits by using 3 bits from the ToS field. (See Figure 3-13.) IP Header Type of Service (ToS) Field This new field is called the Differentiated Services Code Point (DSCP).
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