diff --git a/tftp.c b/tftp.c
index 7009675..6e6aee8 100644
--- a/tftp.c
+++ b/tftp.c
@@ -98,7 +98,7 @@
     return -1;
 }
 
-static int tftp_read_data(struct tftp_session *spt, uint16_t block_nr,
+static int tftp_read_data(struct tftp_session *spt, uint32_t block_nr,
                           uint8_t *buf, int len)
 {
     int bytes_read = 0;
@@ -194,18 +194,14 @@
     tftp_session_terminate(spt);
 }
 
-static int tftp_send_data(struct tftp_session *spt, uint16_t block_nr,
-                          struct tftp_t *recv_tp)
+static int tftp_send_next_block(struct tftp_session *spt,
+                                struct tftp_t *recv_tp)
 {
     struct sockaddr_in saddr, daddr;
     struct mbuf *m;
     struct tftp_t *tp;
     int nobytes;
 
-    if (block_nr < 1) {
-        return -1;
-    }
-
     m = m_get(spt->slirp);
 
     if (!m) {
@@ -219,7 +215,7 @@
     m->m_data += sizeof(struct udpiphdr);
 
     tp->tp_op = htons(TFTP_DATA);
-    tp->x.tp_data.tp_block_nr = htons(block_nr);
+    tp->x.tp_data.tp_block_nr = htons((spt->block_nr + 1) & 0xffff);
 
     saddr.sin_addr = recv_tp->ip.ip_dst;
     saddr.sin_port = recv_tp->udp.uh_dport;
@@ -227,7 +223,7 @@
     daddr.sin_addr = spt->client_ip;
     daddr.sin_port = spt->client_port;
 
-    nobytes = tftp_read_data(spt, block_nr - 1, tp->x.tp_data.tp_buf, 512);
+    nobytes = tftp_read_data(spt, spt->block_nr, tp->x.tp_data.tp_buf, 512);
 
     if (nobytes < 0) {
         m_free(m);
@@ -250,6 +246,7 @@
         tftp_session_terminate(spt);
     }
 
+    spt->block_nr++;
     return 0;
 }
 
@@ -367,7 +364,8 @@
         }
     }
 
-    tftp_send_data(spt, 1, tp);
+    spt->block_nr = 0;
+    tftp_send_next_block(spt, tp);
 }
 
 static void tftp_handle_ack(Slirp *slirp, struct tftp_t *tp, int pktlen)
@@ -380,8 +378,7 @@
         return;
     }
 
-    if (tftp_send_data(&slirp->tftp_sessions[s],
-                       ntohs(tp->x.tp_data.tp_block_nr) + 1, tp) < 0) {
+    if (tftp_send_next_block(&slirp->tftp_sessions[s], tp) < 0) {
         return;
     }
 }
diff --git a/tftp.h b/tftp.h
index b0952e2..c19b159 100644
--- a/tftp.h
+++ b/tftp.h
@@ -37,6 +37,7 @@
 
     struct in_addr client_ip;
     uint16_t client_port;
+    uint32_t block_nr;
 
     int timestamp;
 };