Calculate TCP flag values (SYN, ACK, FIN, RST, PSH, URG) from decimal, hex, or binary. Visualize flag combinations.
Six bits in every TCP header decide whether a packet opens a connection, acknowledges data, or tears it down. This tool takes a TCP flags value in decimal, hex, or binary and returns the other two notations plus the set of flags it encodes - SYN, ACK, FIN, RST, PSH, URG - shown as a labeled bit pattern. The thing people get wrong most often: the flags are numbered from the least significant bit, so SYN is 2 and FIN is 1, and SYN+ACK is 0x12 (18), not 0x03.
A concrete input and expected output from the current implementation.
Input
0x12
Expected output
Decimal 18, hex 0x12, binary 00010010. Flags set: SYN + ACK. Six-bit field (URG ACK PSH RST SYN FIN): 0 1 0 0 1 0.
ACK has weight 16 (bit 4) and SYN weight 2 (bit 1), so the two sum to 18, which is 0x12. The conversion checks out in both directions: the number decodes to exactly those two flags, and adding the two weights reproduces the number.
The six control flags occupy the low six bits of the TCP flags field. Reading from the least significant bit upward the weights are FIN=1, SYN=2, RST=4, PSH=8, ACK=16, URG=32. Any combination equals the sum of its set flags, so one number represents the whole set; the tool splits an entered value into these weights, lists the flags, and converts the value between the three bases.
The result view lays the value out as a fixed-width binary pattern with the six flag labels URG, ACK, PSH, RST, SYN, FIN over the bit positions, set bits highlighted. A value like 0x1B (27) reads at a glance as FIN+SYN+PSH+ACK: bits 0, 1, 3, and 4 set, URG absent. This is the check that catches hand-added sums before they reach a filter or a socket.
The six flags are numbered from the least significant bit, not by prominence or alphabetically: FIN=1, SYN=2, RST=4, PSH=8, ACK=16, URG=32. ACK carries weight 16 and SYN weight 2, so the pair sums to 18. The value 3 is FIN+SYN, a combination associated with connection teardown, not the handshake.
No. This tool covers the six classic control flags, and the ECN bits sit above URG in the real header: ECE=64, CWR=128, NS=256. A capture value like 0x52, which Wireshark renders as SYN+ECE+ACK, is outside this six-flag model, so keep inputs below 64 or account for the ECN bit yourself.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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