In the diagram shown below, L1 is an Ethernet LAN and L2 is a Token-Ring LAN. An IP packet originates from sender S and traverses to R, as shown. The links within each ISP and across the two ISPs, are all point-to-point optical links. The initial value of the TTL field is 32. The maximum possible value of the TTL field when R receives the datagram is ________.

The Time-to-Live (TTL) field in an IPv4 header is decremented by at least $1$ at each router (network layer forwarding device) that processes and forwards the datagram.

Tracing the path from sender $S$ to receiver $R$:

1. Router 1: Gateway connecting LAN L1 to ISP1 (decrements TTL: $32 \to 31$)

2. Router 2: Internal router within ISP1 (decrements TTL: $31 \to 30$)

3. Router 3: Border router of ISP1 (decrements TTL: $30 \to 29$)

4. Router 4: Border router of ISP2 (decrements TTL: $29 \to 28$)

5. Router 5: Internal router within ISP2 (decrements TTL: $28 \to 27$)

6. Router 6: Gateway router connecting ISP2 to LAN L2 (decrements TTL: $27 \to 26$)

From Router 6, the packet is delivered directly across LAN L2 to destination $R$ without passing through any further routers.

Therefore, the maximum possible TTL when $R$ receives the datagram is $32 - 6 = 26$.